USB: serial: mos7840: Fixed MCS7820 device attach problem

USB: serial: mos7840: Fixed MCS7820 device attach problem

commit 093ea2d3a766cb8a4c4de57efec6c0a127a58792 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>