media: davinci_vpfe: get rid of an unused var at dm365_isif.c

Not sure what was the original idea here, but the implementation
went into a different way, and the fmt var is not used
anymore, as warned:

drivers/staging/media/davinci_vpfe/dm365_isif.c: In function '__isif_get_format':
drivers/staging/media/davinci_vpfe/dm365_isif.c:1401:29: warning: variable 'fmt' set but not used [-Wunused-but-set-variable]
   struct v4l2_subdev_format fmt;
                             ^~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>