mmc: renesas_sdhi: refactor calculation of best TAP

To select the best TAP, we need to find the longest stream of set bits
in a bit field. There is now a helper function for bitmaps which
iterates over all region of set bits. Using it makes the code much more
concise and easier to understand. Double so, because we need to handle
two bitmaps in the near future. Remove a superfluous comment while here.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20200408094638.10375-2-wsa+renesas@sang-engineering.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>