net: rtl8139: Split out common and non-DM functions

Split the driver into common and non-DM functionality, so that the
DM support can later re-use the common code, while we retain the
non-DM code until all the platforms are converted.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>