media: i2c: st-vgxy61: Fix smatch warnings

Fix some 'vgxy61_write_reg' return vars not being signed int.
Fix 'expo_long_max' not being initialized.
Handle 'pm_runtime_get_sync' call that can return 1.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>