MIPS: mipsmtregs: Fix target register for MFTC0

Target register of mftc0 should be __res instead of $1, this is
a leftover from old .insn code.

Fixes: dd6d29a6

 ("MIPS: Implement microMIPS MT ASE helpers")
Cc: stable@vger.kernel.org
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>