ethernet: moxa: fix incorrect placement of __initdata tag

__initdata tag should be placed between the variable name and equal
sign for the variable to be placed in the intended .init.data section.

In this particular case __initdata is incorrect as moxart_mac_driver
can be used after the driver gets initialized.

Also while at it static-ize moxart_mac_driver.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>