mmc: sh_mmcif: simplify clock divisor calculation

Replace ilog2(__rounddown_pow_of_two(x)) with the equivalent but much
simpler fls(x) - 1.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>