usb: check device is not NULL before calling usb_ep_get()

In musb_packet(), the call to usb_find_device() can return NULL
if it doesn't find a device matching 'addr' so explicitly check
the return value before passing it to usb_ep_get().  This then
allows the subsequent calculation of 'id' to be streamlined.

Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
Message-id: 1549460216-25808-8-git-send-email-liam.merwick@oracle.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>