ARM: cleanup: debugfs error handling

Debugfs functions return NULL when they fail, or an error pointer
when not configured.  The intention behind the error pointer is that
it appears as a valid pointer to the caller, and so the caller
continues inspite of debugfs not being available.

Debugfs failure should only ever be checked with (!ptr) and not the
IS_ERR*() functions.

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