sfc: Remove versioned bitfield macros

These macros are not extensible to more than two NIC types without
repetition of register definitions, and they are only used to deal
with a few fields in RX_CFG_REG and global events which moved between
Falcon rev A1 and B0.

Therefore:
- Move RX_CFG_REG initialisation into its own function which tests the
  NIC revision just once
- Explicitly test the NIC revision when checking the RX_RECOVERY flag in
  global events
- Merge definitions of RX_XOFF_MAC_EN flag, which did not move
- Remove the macro definitions

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