net: dm9000: Split non-DM specific bits from common code

Split network handling functions into non-DM specific parts and
common code in preparation for conversion to DM.

Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Ramon Fried <rfried.dev@gmail.com>