efi/libstub: Simplify efi_get_memory_map()

Do not check the value of status twice.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Link: https://lore.kernel.org/r/20200216184050.3100-1-xypron.glpk@gmx.de


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>