sfc: Clean up validation of datapath capabilities

Rename efx_ef10_init_capabilities() to the more specific
efx_ef10_init_datapath_caps().

Stop accepting short responses to MC_CMD_GET_CAPABILITIES; we
don't need to support pre-production firmware.

Move the check for RX prefix support from efx_ef10_probe() into
efx_ef10_init_datapath_caps() and use consistent error messages
for missing TSO support and missing RX prefix support.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>