mmc: omap_hsmmc: introduce start_clock and re-use stop_clock

There is similar code in two functions which enable the clock.  Refactor
this code to omap_hsmmc_start_clock(). Re-use omap_hsmmc_stop_clock() in
omap_hsmmc_context_restore() as well.

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>