Blackfin: bf537: rename ad2s120x ->ad2s1200

In v3.2 the Analog Devices ad2s1200/ad2s1205 driver was renamed from
ad2s120x to ad2s1200. But it apparently forgot to rename the references
to this driver in the BF537-STAMP code. Rename these now, and use the
IS_ENABLED() macro, while we're at it.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>