ARM: 6519/1: kuser: Fix incorrect cmpxchg syscall in kuser helpers

The existing code invokes the syscall with rubbish in r7,
due to what looks like an incorrect literal load idiom.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>