net/irda: convert au1k_ir to platform driver.

Moderate driver cleanup:
convert to platform driver, get rid of board-specific code.

Driver loads and runs on a DB1100 board.  But since I have no other
IrDA hardware to exchange data with I can't say whether it really sends
and receives.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: Samuel Ortiz <samuel@sortiz.org>
Cc: netdev@vger.kernel.org
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2877/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>