USB: pl2303: another product ID

I've just got a USB GPRS/EDGE modem branded Manufacturer Micromax Model
MMX610U (see http://www.airtel.in/level2_t3data.aspx?path=1/106/179)
working by adding another product ID to pl2303.  Modem info reports same
module as  Max Arnold's i.e.SIMCOM SIM600  but  with product ID 0x0612
(cf Ox0611).

From: Steve Murphy <steve@gnusis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>