MIPS: unaligned: Fix build error on big endian R6 kernels

Commit eeb53895

 ("MIPS: unaligned: Prevent EVA instructions on kernel
unaligned accesses") renamed the Load* and Store* defines in unaligned.c
to _Load* and _Store* as part of its fix. One define was missed out which
causes big endian R6 kernels to fail to build.

arch/mips/kernel/unaligned.c:880:35:
error: implicit declaration of function '_StoreDW'
 #define StoreDW(addr, value, res) _StoreDW(addr, value, res)
                                   ^

Signed-off-by: James Cowgill <James.Cowgill@imgtec.com>
Fixes: eeb53895

 ("MIPS: unaligned: Prevent EVA instructions on kernel unaligned accesses")
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: <stable@vger.kernel.org> # 4.0+
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10575/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>