MIPS: BMIPS: Fix compilation for BMIPS5000

MIPS: BMIPS: Fix compilation for BMIPS5000

Commit 02b849f7613003fe5f9e58bf233d49b0ebd4a5e8 ("MIPS: Get rid of the
use of .macro in C code.") replaced the macro usage but missed
the accessors in bmips.h, causing the following build error:

  CC      arch/mips/kernel/smp-bmips.o
{standard input}: Assembler messages:
{standard input}:951: Error: Unrecognized opcode `_ssnop'
{standard input}:952: Error: Unrecognized opcode `_ssnop'
(...)
make[6]: *** [arch/mips/kernel/smp-bmips.o] Error 1

Fix by rewriting the inline assembler using existing inline functions.
The generated code should stay unchanged.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/5644/
Reviewed-by: Jonas Gorski <jogo@openwrt.org>