metag: Fix __cmpxchg_u32 asm constraint for CMP

The LNKGET based atomic sequence in __cmpxchg_u32 has slightly incorrect
constraints for the return value which under certain circumstances can
allow an address unit register to be used as the first operand of a CMP
instruction. This isn't a valid instruction however as the encodings
only allow a data unit to be specified. This would result in an
assembler error like the following:

  Error: failed to assemble instruction: "CMP A0.2,D0Ar6"

Fix by changing the constraint from "=&da" (assigned, early clobbered,
data or address unit register) to "=&d" (data unit register only).

The constraint for the second operand, "bd" (an op2 register where op1
is a data unit register and the instruction supports O2R) is already
correct assuming the first operand is a data unit register.

Other cases of CMP in inline asm have had their constraints checked, and
appear to all be fine.

Fixes: 6006c0d8 ("metag: Atomics, locks and bitops")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org
Cc: <stable@vger.kernel.org> # 3.9.x-