USB: serial: add missing MODULE_DESCRIPTION() macros

Since commit 1fffe7a3 ("script: modpost: emit a warning when the
description is missing"), ARCH=x86 make allmodconfig && make W=1 reports:

WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/usb/serial/ch341.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/usb/serial/usb_debug.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/usb/serial/mxuport.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/usb/serial/navman.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/usb/serial/qcaux.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/usb/serial/usb-serial-simple.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/usb/serial/symbolserial.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
[ johan: amend commit message with commit introducing W=1 warning;
         tweak some descriptions ]
Signed-off-by: Johan Hovold <johan@kernel.org>