media: i2c: tc358746: fix ignoring read error in g_register callback

Currently we ignore the return value of tc358746_read() and return
alawys return 0 which is wrong. Fix this by returning the actual return
value of the read operation which is either 0 on success or an error
value.

Addresses-Coverity-ID: 1527254 ("Error handling issues")

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Fixes: 80a21da3

 ("media: tc358746: add Toshiba TC358746 Parallel to CSI-2 bridge driver")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>