sh: fixed cmpxchg gRB version

This patch fixes a bug within the cmpxchg GRB version.

A problem was notices while running some tests to stress
the priority inheritance, for example pi_stress
(http://rt.wiki.kernel.org/index.php/PI_Mutex_Test

).

Also, without this patch, after applying the latest work to
consolidate atomic_cmpxchg() definitions (commit:
8c0b8139)
the Kernel doesn't boot at all.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>