checkpatch: improve UNSPECIFIED_INT test for bare signed/unsigned uses

Improve the test to allow casts to (unsigned) or (signed) to be found
and fixed if desired.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>