sfc: Turn pause frame generation on and off at the MAC, not the RX FIFO

Pause frame generation is gated by both RX_XOFF_MAC_EN and an enable
bit in each MAC.  RX_XOFF_MAC_EN bit always reads back as 0 so we need
to set it correctly every time we modify RX_CFG_REG.  Simplify this by
always setting it to 1 and only changing the enable bits in the MACs.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>