[ARM] Fix stacktrace FP range checking

Fix an oops in the stacktrace code, caused by improper range checking.
We subtract 12 off 'fp' before testing to see if it's below the low
bound.  However, if 'fp' were zero before, it becomes a very large
positive number, causing this test to succeed where it should fail.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>