imxmmc: move RSSR BLR

DMA request source (RSSR) needs to be set only once (in probe).
DMA burst length (BLR) need to be set only in set_ios()

This cleans up imxmci_setup_data() and should make it a little
bit faster :)

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>