net: pcnet: Split common and non-DM functions

Pull the common parts of functions out so they can be reused by both
DM and non-DM code paths. The recv() function had to be reworked to
fit into this scheme and this means it now only receives one packet
at a time instead of spinning in an endless loop.

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