media: staging: media: davinci_vpfe: fix large stack usage with clang

clang is unable to optimize the isif_ioctl() in the same way that
gcc does, as it fails to prove that the local copy of
the 'struct vpfe_isif_raw_config' argument is unnecessary:

drivers/staging/media/davinci_vpfe/dm365_isif.c:622:13: error: stack frame size of 1344 bytes in function 'isif_ioctl' [-Werror,-Wframe-larger-than=]

Marking it as 'const' while passing the data down clearly shows us that
the copy is never modified, and we can skip copying it entirely, which
reduces the stack usage to just eight bytes.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>