Overall, looks pretty clean, good job!

Comments:

1) [major issue] Don't take and release a heavy lock on every single RX 
packet.

2) remove net_device_stats from private structure, and use net_device::stats

3) rx_ring_size should not be a module param, since that should be 
supported via ethtool

4) cpmac_tx_timeout() doesn't really do anything to alleviate the condition

5) don't set dev->mem_start and dev->mem_end, those are fields that are 
going away, and that's not their intended purpose.  just store a pointer 
to the resource info.