r6040: rework the RX buffers allocation routine

Rework the RX buffers allocation function so that we do not
leak memory in the case we could not allocate skbs for the
RX path. Propagate the errors to the r6040_up function
where we call the RX buffers allocation function.

Also rename the r6040_alloc_txbufs function to
r6040_init_txbufs, to reflect what it really does.

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>