usb: musb: jz4740: fix error check of usb_get_phy()

The usb_get_phy() function returns either a valid pointer to phy or
ERR_PTR() error, check for NULL always fails and may lead to oops on
error path, fix this issue.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>