boot1.efi: show EFI error number, not full status value

EFI return values set the high bit to indicate an error. The log
messages changed here are printed only in the case of an error,
so including the error bit is redundant. Also switch to decimal to
match the error definitions (in sys/boot/efi/include/efierr.h).

MFC after: 1 week
Sponsored by: The FreeBSD Foundation