ARM: fix cpu feature extracting helper

Commit b8c9592 "ARM: 8318/1: treat CPU feature register fields as signed
quantities" introduced helper to extract signed quantities of 4-bit
blocks. However, with a current code feature with value 0b1000 isn't
rejected as negative. So fix the "if" condition.

Change-Id: Ia8f9b3e059e7a5a7049ebcd0034a7f7f15174db4
Reported-by: Jonathan Brawn <Jon.Brawn@arm.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Patch-mainline: linux-arm-kernel @ 01/26/16, 10:23
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>