tc35815: Use irq number for tc35815-mac platform device id

The tc35815-mac platform device used a pci bus number and a devfn to
identify its target device, but the pci bus number may vary if some
bus-bridges are found.  Use irq number which is be unique for embedded
controllers.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>