lint: _sbrk_r: change parameter type from size_t to ptrdiff_t

[N8Fear]

incr is of type size_t which is essentially a unsigned int type (according to ISO 1999 C it's an unsigned integer of at least 16 Bit). Since unsigned integers can't be smaller than zero the check is unnecessary (and will hopefully gets optimized away by the compiler anyways).
This PR removes the unnecessary check to silence cppcheck.
This PR changes the type of incr to ptrdiff_t (as suggested by @Kijewski ).
Added: changed this for _sbrk_r implementations for other platforms too.

[Kijewski]

I guess the parameter type for incr is wrong. It should take a ptrdiff_t

[N8Fear]

Changed PR to use ptrdiff_t.

[OlegHahm]

Newlib uses ptrdiff_t, too. Can you change the prototypes for the other platforms, too?

cpu/lpc_common/lpc_syscalls.c:caddr_t _sbrk_r(struct _reent *r, size_t incr)
cpu/mc1322x/mc1322x_syscalls.c:caddr_t _sbrk_r(struct _reent *r, size_t incr)
cpu/nrf51822/syscalls.c:caddr_t _sbrk_r(struct _reent *r, size_t incr)
cpu/sam3x8e/syscalls.c:caddr_t _sbrk_r(struct _reent *r, size_t incr)
cpu/stm32f0/syscalls.c:caddr_t _sbrk_r(struct _reent *r, size_t incr)
cpu/stm32f1/syscalls.c:caddr_t _sbrk_r(struct _reent *r, size_t incr)
cpu/stm32f3/syscalls.c:caddr_t _sbrk_r(struct _reent *r, size_t incr)
cpu/stm32f4/syscalls.c:caddr_t _sbrk_r(struct _reent *r, size_t incr)

[N8Fear]

@OlegHahm: done.
Should I change the title of this PR to reflect this?

[OlegHahm]

Thanks. Changing the title sounds sensible to me.

[Kijewski]

ACK.