pcmcia: pcmcia_dev_present bugfix

pcmcia_dev_present is in and by itself buggy. Add a note specifying
why it is broken, and replace the broken locking -- taking a mutex
is a bad idea in IRQ context, from which this function is rarely
called -- by an atomic_t.

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>