tlan: manage rx allocation failure better

Rx allocation failure at runtime is non-fatal. For normal Rx frame, it
just reuses the buffer, and during setup it just continues with a smaller
receive buffer pool.

Compile tested only.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>