MdeModulePkg/PciBus: Change switch-case to if-else to fix EBC build

EBC compiler doesn't treat EFI_xxx as constant due to these macros
are UINT64 type in 64bit env and UINT32 type in 32bit env.
So it reports error when "case EFI_xxx" is used.
The patch changes to use if-else to fix EBC build failure.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>