metag: Fix atomic_*_return inline asm constraints

The argument i of atomic_*_return() operations is given to inline asm
with the "bd" constraint, which means "An Op2 register where Op1 is a
data unit register and the instruction supports O2R", however Op1 is
constrained by "da" which allows an address unit register to be used.

Fix the constraint to use "br", meaning "An Op2 register and the
instruction supports O2R", i.e. not requiring Op1 to be a data unit
register.

Fixes: d6dfe250 ("locking,arch,metag: Fold atomic_ops")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-metag@vger.kernel.org