USB: serial: mos7840: Fixed MCS7820 device attach problem

A MCS7820 device supports two serial ports and a MCS7840 device supports
four serial ports. Both devices use the same driver, but the attach function
in driver was unable to correctly handle the port numbers for MCS7820
device. This problem has been fixed in this patch and this fix has been
verified on x86 Linux kernel 3.2.9 with both MCS7820 and MCS7840 devices.

Signed-off-by: Donald Lee <donald@asix.com.tw>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>