usb: musb: gadget: fix to_musb_ep() to not return NULL

UDC core ensures the usb_ep parameter passed in is not NULL, so
checking if (ep != NULL) is pointless.

Convert to_musb_ep() to a simple macro to not directly return NULL to
avoid warnings from code static analysis tools.

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