net: pcnet: Simplify private data allocation

The current code is horribly complex. Both the RX and TX buffer
descriptors are 16 bytes in size, the init block is 32 bytes in
size, so simplify the code such that the entire private data of
the driver are allocated cache aligned and the RX and TX buffer
descriptors are part of the private data.

This removes multiple malloc calls and cache flushes.

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