cxgb4: fix wrong shift.

While fixing coverity warning, commit dd2c7967 introduced typo in
shift value. Fix that.

Signed-off-by: Pavel Machek (CIP) <pavel@denx.de>
Fixes: dd2c7967 ("cxgb4: Fix unintentional sign extension issues")
Signed-off-by: David S. Miller <davem@davemloft.net>