media: ov2740: only do OTP data read on demand from user

OTP data access of ov2740 in probe need power up, it may cause
the camera flash LED blink during probe if the LED use same power
rail with camera, this patch move the OTP data access out of
probe, it will only occur on demand from user by nvmem sysfs.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Qingwu Zhang <qingwu.zhang@intel.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>