mmc: Add helper function to check if a card is removable

There are two checks that need to be made when determining whether a
card is removable. A host controller may set MMC_CAP_NONREMOVABLE if the
controller does not support removing cards (e.g. eMMC), in which case
the card is physically non-removable. Also the 'mmc_assume_removable'
module parameter can be configured at module load time, in which case
the card may be logically non-removable.

A helper function keeps the logic in one place so that code always
checks both conditions.

Because this new function is likely to be called from modules we now
need to export the mmc_assume_removable symbol.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>