Fixed a compilation error, with irq debugging for stm32f7 and stm32h7 archs.

[wingunder]

This commit fixes a compilation error that occurs when enabling the
following configuration items for stm32f7 and stm32h7 architectures:

   CONFIG_DEBUG_FEATURES=y
   CONFIG_DEBUG_ERROR=y
   CONFIG_DEBUG_WARN=y
   CONFIG_DEBUG_INFO=y
   CONFIG_DEBUG_IRQ=y
   CONFIG_DEBUG_IRQ_ERROR=y
   CONFIG_DEBUG_IRQ_WARN=y
   CONFIG_DEBUG_IRQ_INFO=y

The compiler error for stm32f7:

make[1]: Entering directory '/home/pdupreez/dev/wingunder/nuttx/arch/arm/src'
CC:  chip/stm32_irq.c
chip/stm32_irq.c: In function 'up_irqinitialize':
chip/stm32_irq.c:497:29: error: 'STM32_IRQ_NIRQS' undeclared (first use in this function); did you mean 'STM32_IRQ_FIRST'?
   stm32_dumpnvic("initial", STM32_IRQ_NIRQS);
                                ^~~~~~~~~~~~~~~
                             STM32_IRQ_FIRST
chip/stm32_irq.c:497:29: note: each undeclared identifier is reported only once for each function it appears in
make[1]: *** [Makefile:172: stm32_irq.o] Error 1
make[1]: Leaving directory '/home/pdupreez/dev/wingunder/nuttx/arch/arm/src'

And the compiler error for stm32h7:

make[1]: Entering directory '/home/pdupreez/dev/wingunder/nuttx/arch/arm/src'
CC:  chip/stm32_irq.c
chip/stm32_irq.c: In function 'stm32_dumpnvic':
chip/stm32_irq.c:164:4: warning: #warning Missing logic [-Wcpp]
 #  warning Missing logic
     ^~~~~~~
     chip/stm32_irq.c: In function 'up_irqinitialize':
     chip/stm32_irq.c:522:29: error: 'STM32_IRQ_NIRQS' undeclared (first use in this function); did you mean 'STM32_IRQ_CRS'?
        stm32_dumpnvic("initial", STM32_IRQ_NIRQS);
                                     ^~~~~~~~~~~~~~~
                                  STM32_IRQ_CRS
chip/stm32_irq.c:522:29: note: each undeclared identifier is reported only once for each function it appears in
make[1]: *** [Makefile:172: stm32_irq.o] Error 1
make[1]: Leaving directory '/home/pdupreez/dev/wingunder/nuttx/arch/arm/src'

This commit replaces all STM32_IRQ_NIRQS defines with the NR_IRQS
define, which seems to be consistent with the rest of the code in
Nuttx.