Revert "mmc: block: don't use parameter prefix if built as module"

This reverts commit 829b6962.

Andorid userspace looks for the path:
/sys/module/mmcblk/parameters/perdev_minors

But with this change, that path has moved to:
/sys/module/mmc_block/parameters/perdev_minors

This seems like a kernel ABI regression, so this patch should
likely be reverted.

Signed-off-by: John Stultz <john.stultz@linaro.org>