sfc: allow ethtool selftest and MC reboot to complete on an unprivileged function

The policy in the net driver is to attempt MCDI commands and
then handle any EPERM error codes appropriately when returned
by unprivileged functions.
The ethtool selftest contains some tests which are useful on
an unprivileged function, such as the event queue interrupt
tests, but other tests cannot be performed as the function
does not have the required permissions.

If a test returns -EPERM, act as though the test was not run
and continue.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>