mmc: omap_hsmmc: Get rid of omap_hsmmc_4_set_power function

Now that omap_hsmmc_set_power() already has a check to return 0
if !host->vcc, it seems like it can be used even on MMC4 instead
of the dummy omap_hsmmc_4_set_power().

This also helps get rid of all the host->id based check to
populate the right function for on-chip/external level
shifting and use omap_hsmmc_set_power() for all MMC modules.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Tested-by: Venkatraman S <svenkatr@ti.com>
Tested-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>