media: platform: ti: Add missing check for devm_regulator_get

Add check for the return value of devm_regulator_get since it may return
error pointer.

Fixes: 448de7e7 ("[media] omap3isp: OMAP3 ISP core")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>