pcmcia: at91_cf: make mc static

The symbol is not used outside of the file, so mark it static.

Fixes the following warning:

./drivers/pcmcia/at91_cf.c:49:15: warning: symbol 'mc' was not declared. Should it be static?

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