serial: add PNP ID for Davicom ISA 33.6K modem

This should resolve these bug reports of the modem not working:
    http://bugzilla.kernel.org/show_bug.cgi?id=4355
    http://www.linuxquestions.org/questions/linux-newbie-8/connect-script-failed-on-ppp-go-123975/



I don't have hardware to test this, but the initial report in the kernel
bugzilla indicates that this change fixed the problem.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Dmitry Vavilov <vavilov@ihep.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>