mmc: renesas_sdhi: replace EXT_ACC with HOST_MODE

All our documentation says HOST_MODE, we don't really know where EXT_ACC
came from. Rename it to reduce the confusion.

Signed-off-by: Masaharu Hayakawa <masaharu.hayakawa.ry@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>