usb: musb: blackfin: add bfin_fifo_offset in bfin_ops

The function bfin_fifo_offset is defined but not used:

drivers/usb/musb/blackfin.c:36:12: warning: ‘bfin_fifo_offset’ defined
but not used [-Wunused-function]
 static u32 bfin_fifo_offset(u8 epnum)
             ^~~~~~~~~~~~~~~~

Adding bfin_fifo_offset to bfin_ops fixes this warning and allows musb
core to call this function instead of default_fifo_offset.

Fixes: cc92f681 ("usb: musb: Populate new IO functions for blackfin")
Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>