fix(sdei): resolve coverity error for SDEI GIC

Fix the following issues from Coverity check:

*** CID 463142:  Memory - illegal accesses  (INTEGER_OVERFLOW)
/services/std_svc/sdei/sdei_event.c: 79 in get_event_entry_target_pe()

*** CID 463141:  Memory - illegal accesses  (NEGATIVE_RETURNS)
/services/std_svc/sdei/sdei_intr_mgmt.c: 64 in sdei_is_target_pe_masked()

Change-Id: I0f2928a6405f091b1ea988a43428d7032ee4c136
Signed-off-by: Mark Dykes <mark.dykes@arm.com>
3 files changed