MMC: at91 mmc linkage updates

Linker level tweaks for the AT91 MMC driver:

  - fix a wrongly-exported symbol
  - move probe() to init section
  - move remove() to exit section

When this driver is statically linked, this patch shrinks the driver's
runtime I-space footprint by over 20% (950 bytes).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>