usb: gadget: mv_udc: fix the value of tranceiver

usally we will use udc->tranceiver == NULL or
udc->tranceiver != NULL.
So when failed to get the udc->tranceiver by usb_get_phy(), we
directly set udc->tranceiver to be NULL.
Then the source code will not need macro IS_ERR_OR_NULL() for
udc->tranceiver judgement. It can reduce the line size and make
the judgement simple.

Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>