ArmPkg: Fix uninitialised variable in ArmMmuStandaloneMmLib

The following patches added support for StandaloneMM using FF-A:
9da5ee1 ArmPkg: Allow FF-A calls to set memory region's attributes
0e43e02 ArmPkg: Allow FF-A calls to get memory region's attributes

However, in the error handling logic for the Get/Set Memory attributes,
the CLANG compiler reports that a status variable could be used without
initialisation. This issue is a false positive and is not seen with GCC.

The Get/Set Memory attributes operation is atomic and therefore an
FFA_INTERRUPT or FFA_SUCCESS response is not expected in response
to FFA_MSG_SEND_DIRECT_REQ. So the remaining cases that could occur
are:
 - the target sends FFA_MSG_SEND_DIRECT_RESP with a success or
   failure code.
 or
 - FFA_MSG_SEND_DIRECT_REQ transmission failure.

Therefore,
 - reorder the error handling conditions such that it prevents the
   uninitialised variable issue being flagged by CLANG.
 - move the repetitive code to a static helper function and add
   documentation at the appropriate places.
 - fix error handling in functions that invoke GetMemoryPermissions().

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Tested-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Sughosh Ganu <sughosh.ganu@linaro.org>

Author: samimujawar