USB: f_ncm: ncm_bitrate (speed) is unsigned

USB: f_ncm: ncm_bitrate (speed) is unsigned

[  190.544755] configfs-gadget gadget: notify speed -44967296

This is because 4250000000 - 2**32 is -44967296.

Fixes: 9f6ce4240a2b ("usb: gadget: f_ncm.c added")
Cc: Brooke Basile <brookebasile@gmail.com>
Cc: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Yauheni Kaliuta <yauheni.kaliuta@nokia.com>
Cc: Linux USB Mailing List <linux-usb@vger.kernel.org>
Acked-By: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210608005344.3762668-1-zenczykowski@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>