m68k: mac: Remove forward declaration for mac_nmi_handler()

Move mac_nmi_handler() before its caller, so its forward declaration can
be dropped.  While at it, make the function static, and absorb the
variable in_nmi, which is only used inside the function.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Finn Thain <fthain@linux-m68k.org>
Link: https://lore.kernel.org/r/fa48394ecba7c29a2da354d13c1017b37237f25b.1657114783.git.geert@linux-m68k.org