sfc: Treat all MAC registers as 128-bit

Although all the defined fields in these registers are within 32 bits,
they are architecturally defined as 128-bit like most other Falcon
registers.  In particular, we must use efx_reado() to ensure proper
locking when reading MD_STAT_REG.

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