arm: fix implicit use of sched.h in bcmring/dma.c

To fix this:

arch/arm/mach-bcmring/dma.c: In function 'dma_request_channel_dbg':
arch/arm/mach-bcmring/dma.c:1022: error: 'TASK_INTERRUPTIBLE' undeclared (first use in this function)
arch/arm/mach-bcmring/dma.c:1022: error: (Each undeclared identifier is reported only once

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>