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 <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/m68k/atari/atakeyb.c

···
 }
 
 /* Reset (without touching the clock) */
-void ikbd_reset(void)
+static void ikbd_reset(void)
 {
 	static const char cmd[2] = { 0x80, 0x01 };