sfc: cope with ENOSYS from efx_mcdi_get_workarounds()

GET_WORKAROUNDS was only introduced in May 2014, not all firmware
 will have it.  So call sites need to handle ENOSYS.
In this case we're probing the bug26807 workaround, which is not
 implemented in any firmware that doesn't have GET_WORKAROUNDS.
 So interpret ENOSYS as 'false'.

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