NET: sa11x0-ir: add DMA support for SIR transmit mode

As the DMA engine API allows DMA channels to be reconfigured on the
fly, we can now support switching the DMA channel configuration to
support SIR transmit DMA without needing to claim an additional
physical DMA channel - thereby using up half the DMA channels just
for one driver.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>