cxgb4: exclude registers with read side effects from register dumps

A few registers have side effects on reads, eg FIFO pointers that
auto advance or mailboxes which change ownership when read.  They are
unsafe to read so exclude them from ethtool register dumps.

Bump ethtool_regs.version to indicate the changed register set.

Signed-off-by: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>