USB: serial: cypress_m8: fix interrupt-out transfer length

Fix interrupt-out transfer length which was being set to the
transfer-buffer length rather than the size of the outgoing packet.

Note that no slab data was leaked as the whole transfer buffer is always
cleared before each transfer.

Fixes: 9aa8dae7 ("cypress_m8: use usb_fill_int_urb where appropriate")
Signed-off-by: Johan Hovold <johan@kernel.org>