USB: usbserial: mos7720: cleanup, consolidation, replace send_mos_cmd with {read,write}_mos_reg

No functionality added or bugs fixed, just improved code consistency and
(hopefully) readability by replacing send_mos_cmd with the register read & write
functions that were used for parallel port registers.  Also shortens overall
file length.

Thoroughly tested, with emphasis on regression testing the serial port.
Signed-off-by: NMike Dunn <mikedunn@newsguy.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>