r6040: completely rework the RX path

This patch completely reworks the RX path in order to be
more accurate about what is going on with the MAC.
We no longer read the error from the MLSR register instead read
the descriptor status register which reflects, the error per descriptor.
We now allocate skbs on the fly in r6040_rx, and we handle allocation
failure instead of simply dropping the packet. Remove the
rx_free_desc counter of r6040_private structure since we
allocate skbs in the RX path.

r6040_rx_buf_alloc is now removed and becomes unuseless.

Signed-Off-By: Joerg Albert <jal2@gmx.de>
Signed-off-by: Florian Fainelli <florian.fainelli@telecomint.eu>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>