USB: serial: mct_u232: added _ioctl, _msr_to_icount and _get_icount functions

Added mct_u232_ioctl (implements TIOCMIWAIT command),
mct_u232_get_icount (implements TIOCGICOUNT command) and
mct_u232_msr_to_icount functions. MCT U232 P9 is one of a few usb to
serail adapters which converts USB +/-5v voltage levels to COM +/-15
voltages. So it can also power COM interfaced devices. This makes it
very usable for legacy COM interfaced data-acquisition hardware. I
tested new implementation with AWARE Electronics RM-60 radiation meter,
which sends pulse via RNG COM line whenever new particle is registered.

Signed-off-by: Vadim Tsozik <tsozik@yahoo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>