USB: ehci-msm: Avoid possible null pointer dereference

devm_usb_get_phy can return null and value retuned by it is used in
otg_se_host so handle if devm_usb_get_phy retuns null and avoid null
pointer dereference.

CRs-fixed: 562868
Change-Id: Ic9e4069bf78254072bb07a6fc4d2c014b1cb8708
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>