mmc: omap_hsmmc: call omap_hsmmc_set_power directly

If no pdata.set_power was set by the platform code, the driver
was updating pdata with its own fallback function. This is a no-no
since pdata shall be read-only.
This patch pushes the check 'pdata->set_power != NULL' down into
the fallback functions. If pdata.set_power is really set, it calls them
and exits, otherwise the fallback code is used.

Signed-off-by: Andreas Fenkart <afenkart@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>