mtd: maps: Blackfin async: rename local funcs to avoid common clashes

There are new Blackfin MMR helper functions that use the same name as some
of the local functions in this driver, so have the driver use more specific
names to avoid the issue.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>