m68k: atari: Make ikbd_reset() static

m68k: atari: Make ikbd_reset() static

When building with W=1:

    arch/m68k/atari/atakeyb.c:335:6: warning: no previous prototype for ‘ikbd_reset’ [-Wmissing-prototypes]
      335 | void ikbd_reset(void)
	  |      ^~~~~~~~~~

Fix this by making ikbd_reset() static.
There was never a user outside arch/m68k/atari/atakey.c.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/6babf691fff55f913808ad845e66f60ab00063b2.1694613528.git.geert@linux-m68k.org