[media] media: i2c: lm3560: fix missing unlock on error, fault handling

Correct reference of reading values. (rval -> reg_val)
Add the missing unlock before return from function lm3560_get_ctrl()
to avoid deadlock.
Thank you Dan Carpenter & Sakari.

Signed-off-by: Daniel Jeong <gshark.jeong@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>