usb: gadget: u_serial.h: increase MAX_U_SERIAL_PORTS to 8

Mediatek's LTE modem needs up to 8 ports to connect to PC for logging
and debugging under some scenarios. Hence we suggest to increase the
definition of MAX_U_SERIAL_PORTS to 8 for some complex embedded systems.

Signed-off-by: Macpaul Lin <macpaul.lin@mediatek.com>
Link: https://lore.kernel.org/r/1592286977-30483-1-git-send-email-macpaul.lin@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>