net: 8390: Fix manufacturer name in Kconfig help text

The help text refers to Western Digital instead of National
Semiconductor 8390, presumably because it was copied from the former.

Fixes: 644570b8

 ("8390: Move the 8390 related drivers")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>