omap_hsmmc: Fix MMC3 dma

Data transfers on third OMAP3 MMC controller don't work
because DMA line numbers are only defined for MMC1 and MMC2.
Fix that and store line numbers in mmc_omap_host structure
to reduce code size.
Tested on OMAP3 pandora board.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>