Platform: OLPC: Use BIT() and GENMASK() for event masks

Just a cosmetic tidy-up.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>