Out of the box Ubuntu's 20.04 compiler warns about missing return value
checks for write() (sys)calls.

Make GCC happy by checking whether we actually managed to write "val".
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-5-andre.przywara@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>