[media] em28xx: reduce stack usage in sensor probing functions

It's no longer necessary to keep the i2c_client in the device struct
unmodified until a sensor is found, so reduce stack usage in
em28xx_probe_sensor_micron() and em28xx_probe_sensor_omnivision() by using
a pointer to the client instead of a local copy.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>