m68knommu: fix IO write size in nettel pin set

The pin write code that supports the UART signals is not using he correct
word write IO access method. It correctly reads the correct 16 bit
registrer, it should also write the new value back with a 16 bit write.
Fix it to use writew().

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>