MIPS: mips-r2-to-r6-emul.h: Inline empty `mipsr2_decoder'

MIPS: mips-r2-to-r6-emul.h: Inline empty `mipsr2_decoder'

Use `static inline' rather than `static __maybe_unused' for
`mipsr2_decoder' in the empty case, making inlining explicit where it
will happen anyway.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9678/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>