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

The gadget function drivers should ensure the usb_request parameter
passed in is not NULL. UDC core doesn't check if it is NULL, so MUSB
driver shouldn't have to check it either.

Convert to_musb_request() 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>