arm: tegra: usb_phy: set vbus_reg to NULL

Setting vbus_reg to NULL if regulator get
fails.

Bug 1047065

Change-Id: I574f58d46f226d70034a4f363103fd9763482292
Signed-off-by: Petlozu Pravareshwar <petlozup@nvidia.com>
Reviewed-on: http://git-master/r/131759
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Venkat Moganty <vmoganty@nvidia.com>