edac: cortex_arm64: Remove misleading edac error warning

If bad_mode() or do_bad() is called, the arm edac driver checks for an
error. Ensure that warning messages are only printed out if there is
actually an error.

Additionally, fix an issue where the warning for a single bit error could
be printed for a double bit error.

Change-Id: I6133cb298fb9e660a220434761427d6ea6adb2ba
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>