mmc: renesas_sdhi: clarify handling of selecting TAPs

mmc: renesas_sdhi: clarify handling of selecting TAPs

The comment and the define about how TAPs are selected were confusing to
me because the good TAP was only valid if it was bigger than a *_MAX_*
value. Rename the define and adapt the comment to what really happens.

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/20200408094638.10375-3-wsa+renesas@sang-engineering.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>