mmc: core: Provide description for sdio_set_host_pm_flags()'s 'flag' arg

Add missing kerneldoc entry for sdio_set_host_pm_flags()'s 'flag'
function argument.

Fixes the following W=1 kernel build warning:

 kernel/drivers/mmc/core/sdio_io.c:722: warning: Function parameter or member 'flags' not described in 'sdio_set_host_pm_flags'

Cc: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200701124702.908713-3-lee.jones@linaro.org


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