net: w5100: move mmiowb into register access callbacks

Instead of sprinkle mmiowb over the driver code, move it into primary
register write callbacks. (w5100_write, w5100_write16, w5100_writebuf)

This is a preparation for supporting SPI interface which doesn't use
MMIO for accessing w5100 registers.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Mike Sinkovsky <msink@permonline.ru>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>