usb: dc_dw: Fix FIFO available space

For now tx-fifo sizes are not configured (cf usb_dw_set_fifo).
Default fifo size is 136 bytes and sending more than 136 bytes
makes the data split into incorrect chunk size.
This patch prevents this by reducing available fifo size to a
multiple of the endpoint max packet size.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>