m68k: bvme6000: Make bvme6000_abort_int() static

When building with W=1:

    arch/m68k/bvme6000/config.c:133:13: warning: no previous prototype for ‘bvme6000_abort_int’ [-Wmissing-prototypes]
      133 | irqreturn_t bvme6000_abort_int (int irq, void *dev_id)
	  |             ^~~~~~~~~~~~~~~~~~

Fix this by making bvme6000_abort_int() static.
There was never a user outside arch/m68k/bvme6000/config.c.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/4da40748e1d7d56b73840a1d7ab1cb9c9a59cef2.1694613528.git.geert@linux-m68k.org