ATM, Solos PCI ADSL2+: Don't deref NULL pointer if net_ratelimit() and alloc_skb() interact badly.

If alloc_skb() fails to allocate memory and returns NULL then we want to
return -ENOMEM from drivers/atm/solos-pci.c::popen() regardless of the
value of net_ratelimit(). The way the code is today, we may not return if
net_ratelimit() returns 0, then we'll proceed to pass a NULL pointer to
skb_put() which will blow up in our face.
This patch ensures that we always return -ENOMEM on alloc_skb() failure
and only let the dev_warn() be controlled by the value of net_ratelimit().

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David S. Miller <davem@davemloft.net>