media: i2c: tc358746: fix possible endianness issue

Using the u64 v4l2_dbg_register.val directly can lead to unexpected
results depending on machine endianness. Fix this by using a local
variable which is assigned afterwards. Since tc358746_read() will init
the val variable to 0 we can assing it without checking the return value
first.

Addresses-Coverity-ID: 1527256 ("Integer 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>