mmc: don't use weight32()

Using weight32() to determine if a value is a power of 2 is a rather
heavi weight solution.  The classic idiom is (x & (x - 1)) == 0, but
the kernel already provide a is_power_of_2 function for it.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>