USB: serial: allow up to 16 ports per device

Raise the arbitrary limit of how many ports a single device can claim
from eight to 16.

This specifically enables the upper eight ports of some mxuport devices.
Signed-off-by: NJohan Hovold <johan@kernel.org>

include/linux/usb/serial.h

@@ -20,7 +20,7 @@
 #include <linux/kfifo.h>
 
 /* The maximum number of ports one device can grab at once */
-#define MAX_NUM_PORTS		8
+#define MAX_NUM_PORTS		16
 
 /* parity check flag */
 #define RELEVANT_IFLAG(iflag)	(iflag & (IGNBRK|BRKINT|IGNPAR|PARMRK|INPCK))