sfc: Explain why efx_mcdi_exit_assertion() ignores result of efx_mcdi_rpc()

Fix CID 113952 in Coverity report on Linux.

This is the one instance where we don't, and shouldn't, check the
return code from efx_mcdi_rpc().  It wasn't immediately obvious to me
why we didn't, so I think an explanation is in order.

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