vfio: remove unneeded test

clang complained that the comparison of an u8 variable against 256 is
somewhat pointless.

Just remove the check, as the condition will never hit.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>