sh_eth: add sh_eth_cpu_data::no_tx_cntrs flag

RZ/A1H (R7S72100) Ether controller doesn't  seem to have the TX counter
registers like TROCR/CDCR/LCCR (or at least they are still undocumented
like some TSU registers), so we bail out of sh_eth_get_stats() early in
this case.  Currently we are calling sh_eth_is_rz_fast_ether() in order
to check for this, but it would be simpler to check the new 'no_tx_cntrs'
bitfield in the 'struct sh_eth_cpu_data'; then we'd be able  to remove
sh_eth_is_rz_fast_ether() as there would be no callers left...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>