alx: refactor descriptor allocation

Split the allocation of descriptor memory and the buffer allocation into a
tx and rx function. This is in preparation for multiple queues where we
need to iterate over the new functions.

While at it drop the unneeded casting on the rx side.

Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>