ARM: OMAP: MUSB: DMA interrupt locking fix

dma_controller_irq() should take the lock (and disable interrupts) as
the main interrupt handler does since the functions it calls assume
this to be the case.

Signed-off-by: Kevin Hilman <khilman@mvista.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>