mmc: tmio: refactor CLK_CTL bit calculation

for (clk = 0x80000080; new_clock >= (clock << 1); clk >>= 1)
          clock <<= 1;

... is too tricky, hence I replaced with

  roundup_pow_of_two(divisor) >> 2

'(clk >> 22) & 0x1' is the bit test for the 1/1 divisor, but
it is not clear.  'divisor <= 1' is easier to understand.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>