mmc: renesas_sdhi: improve HOST_MODE usage

HOST_MODE should have a CTL_ prefix, too. This makes the code more
readable because we immediately know what it is. Also, remove the
hardcoded values with something readable, too.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20201111100244.15823-2-wsa+renesas@sang-engineering.com


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