mmc: omap_hsmmc: move hardcoded frequency constants to defines

Move the min and max frequency constants to the definition block in
the source file.

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>