arm: tegra: usb_phy: fix null pointer dereference

Fixing the null pointer dereference crash. Also
modifying USB driver not to give up in case if there
is no avdd_usb regulator.

Bug 1029852

Change-Id: I3232b7fa1580a0dc1aadd47b138e5eeae05af6f0
Signed-off-by: Rakesh Bodla <rbodla@nvidia.com>
Reviewed-on: http://git-master/r/121984
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Venkat Moganty <vmoganty@nvidia.com>
(cherry picked from commit 48d8c7c91ea5acddf23f74abc801c89c9f16680f)
Reviewed-on: http://git-master/r/123486
Reviewed-by: Simone Willett <swillett@nvidia.com>
Tested-by: Simone Willett <swillett@nvidia.com>