efi: Use NULL instead of 0 for pointer

Fix following sparse warnings:

drivers/firmware/efi/efivars.c:230:66: warning:
 Using plain integer as NULL pointer
drivers/firmware/efi/efi.c:236:27: warning:
 Using plain integer as NULL pointer

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>