m68k: Add missing ioport_unmap()

drivers/net/ethernet/cirrus/cs89x0.c: In function ‘cs89x0_ioport_probe’:
drivers/net/ethernet/cirrus/cs89x0.c:1629: error: implicit declaration of function ‘ioport_unmap’

Add the missing ioport_unmap() implementation, and convert ioport_map()
from a macro to a static inline function while we're at it (both copied
from asm-generic).

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>