sfc: determine flag word automatically in efx_has_cap()

Now that we have an _OFST definition for each individual flag bit,
 callers of efx_has_cap() don't need to specify which flag word it's
 in; we can just use the flag name directly in MCDI_CAPABILITY_OFST.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>