usb: phy: protect against NULL phy pointers

In order to decrease the amount of work done
by PHY users, allow NULL phy pointers to be
passed.

Change-Id: Ied77ecc9923f1d2082ad2a70f64034d8fa04c6c4
Signed-off-by: Felipe Balbi <balbi@ti.com>
Git-commit: 410aee70f06da60dd8f78c3ec3346cc74c9bed46
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Jack Pham <jackp@codeaurora.org>