MIPS: ralink: Remove ralink_halt()

ralink_halt() does nothing that machine_halt() doesn't already do, so it
adds no value.

It actually causes incorrect behaviour due to the "unreachable()" at the
end. This tells the compiler that the end of the function will never be
reached, which isn't true. The compiler responds by not adding a
'return' instruction, so control simply moves on to whatever bytes come
afterwards in memory. In my tested, that was the ralink_restart()
function. This means that an attempt to 'halt' the machine would
actually cause a reboot.

So remove ralink_halt() so that a 'halt' really does halt.

Fixes: c06e836a

 ("MIPS: ralink: adds reset code")
Signed-off-by: NeilBrown <neil@brown.name>
Cc: John Crispin <john@phrozen.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 3.9+
Patchwork: https://patchwork.linux-mips.org/patch/18851/
Signed-off-by: James Hogan <jhogan@kernel.org>