Blackfin: isram: clean up ITEST_COMMAND macro and improve the selftests

The IADDR2DTEST() macro had some duplicated logic with bit 11 and some
incorrect comments, so scrub all of that.

In order to verify these aren't a problem (and won't be in the future),
extend the self tests to operate on as much L1 SRAM as possible.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>