mmc: Fix omap compile by replacing dev_name with dma_dev_name

This patch fixes error:

drivers/mmc/host/omap.c: In function 'mmc_omap_get_dma_channel':
drivers/mmc/host/omap.c:1038: error: called object 'dev_name' is not a function

Commit 06916639

 adds a function
called dev_name. This will cause a name conflict as dev_dbg calls
dev_name(((host->mmc)->parent)).

This same issue should not affect other drivers as they don't seem
to use dev_name with dev_dbg.

Thanks to Paul Walmsley <paul@pwsan.com> for figuring this one out.

Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>