mmc: sdio: Tidy error path in mmc_attach_sdio()

The error path deletes the device by calling mmc_sdio_remove() which must
be called without the host claimed. Simplify the error path so it does just
that and add a comment about why we don't disable runtime PM.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>