media: am437x: fix smatch warning

Fixes this warning:

drivers/media/platform/am437x/am437x-vpfe.c:288 vpfe_ccdc_validate_param() warn: unsigned 'ccdcparam->alaw.gamma_wd' is never less than zero.

by dropping the gamma_wd < VPFE_CCDC_GAMMA_BITS_15_6 check
since VPFE_CCDC_GAMMA_BITS_15_6 is 0.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>