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

It's excessive to use prefix for the parameters when you do
modprobe mmc-block mmcblk.perdev_minors=16

Make this available only for built-in case.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>