net: pcnet: Move private data allocation to initialize

The private data allocation does not have to be done every time the
NIC is initialized at run time, move the allocation to initialize
function, which means it will be done only once when the driver
starts.

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