ARM: McBSP: Fix request for irq in OMAP4

In OMAP4, there is only one irq line for TX and RX paths. Use
the correct irq line to avoid errors at runtime.

Also, request irq line only once (instead of requesting for TX
and RX).

Signed-off-by: Jorge Eduardo Candelaria <jorge.candelaria@ti.com>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>