usb: netusb: Align NETUSB_MTU & wMaxSegmentSize and increase it to 1522

As rx_buf & tx_buf are defined with a NETUSB_MTU size, this cause
that the frames over 1500 bytes can not be received and are discarded.

Secondly, as fragmentation is not supported (for UDP and TCP datagram),
increase NETUSB_MTU to the maximum possible ethernet frame size :1522

Signed-off-by: Nicolas LANTZ <nicolas.lantz@ubicore.net>