media: i2c: vd55g1: Use first index of mbus codes array as default

media: i2c: vd55g1: Use first index of mbus codes array as default

Factorize code and prevent future errors in case of media bus codes
change. Rename VD55G1_DEFAULT_MODE to VD55G1_MODE_DEF to mimic other
macros and reduce vd55g1_update_img_pad_format() name to
vd55g1_update_pad_fmt() to stay within the 80 characters limit.

Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>