media: imx274: rename frmfmt and format to "mode"

A mix of "mode", "format" and "frmfmt" is used to refer to the sensor
readout mode. Use the term "mode" for all of them. Now "format" is
only used in the V4L2 meaning.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>