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

This reverts commit 829b6962.

Revert this change as it causes a sysfs path to change and therefore
introduces and ABI regression. More precisely Android's vold is not being
able to access /sys/module/mmcblk/parameters/perdev_minors any more, since
the path becomes changed to: "/sys/module/mmc_block/..."

Fixes: 829b6962 ("mmc: block: don't use parameter prefix if built as
module")
Reported-by: John Stultz <john.stultz@linaro.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>