mmc: core: Improve documentation of MMC_CAP_HW_RESET

MMC_CAP_HW_RESET means that the controller is capable of resetting the eMMC
device via RST_n signal, not a reset of the controller. Two drivers got
this wrong, so let's make it more clear.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20200821063533.3771-1-wsa+renesas@sang-engineering.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>