SDIO functions are initialized with a disabled runtime PM state,
and are set active (and their usage count is incremented)
only before potential drivers are probed.
SDIO function drivers that support runtime PM should call
pm_runtime_put_noidle() in their probe routine, and
pm_runtime_get_noresume() in their remove routine (very
similarly to PCI drivers).
In case a matching driver does not support runtime PM, power will
always be kept high (since the usage count is positive).