checkpatch: allow parentheses on return for comparisons

It seems to be a common idiom to include braces on conditionals in all
contexts including return.  Allow this exception to the return is not a
function checks.  Reported by Kay Sievers.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>