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>

drivers/pcmcia/at91_cf.c

@@ -46,7 +46,7 @@ struct at91_cf_data {
 #define AT91_IDE_SWAP_A0_A2	0x02
 };
 
-struct regmap *mc;
+static struct regmap *mc;
 
 /*--------------------------------------------------------------------------*/