sh: traps: Make is_dsp_inst() static

If CONFIG_SH_DSP=y (e.g. se7343_defconfig):

    arch/sh/kernel/traps_32.c:572:5: warning: no previous prototype for ‘is_dsp_inst’ [-Wmissing-prototypes]

There are no users outside this file, so make it static.

While at it, convert the dummy for the CONFIG_SH_DSP=n case from a macro
to a static inline function, to increase type-safety.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/8525fe446e7f24649a83b8cd6ca8b736ab746b80.1709579038.git.geert+renesas@glider.be


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>