mtd: clean up usage of MTD_DOCPROBE_ADDRESS

Depending on whether MTD_DOCPROBE_ADVANCED is set or not,
MTD_DOCPROBE_ADDRESS will default to either 0x0000 or 0. That should
lead to (basically) identical code in docprobe.c. The current two
defaults should be merged.

And, while we're at it, if MTD_DOCPROBE is set MTD_DOCPROBE_ADDRESS will
always be set. (MTD_DOCPROBE_ADDRESS depends on MTD_DOCPROBE and it has
a default value.) So the check whether CONFIG_MTD_DOCPROBE_ADDRESS is
defined is unnecessary and should be dropped.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>