sfc: Do not initialise buffer in efx_alloc_special_buffer()

Currently we initialise the newly allocated buffer to all-1s, which is
important for event queues but not for descriptor queues.  And since
we also do that in efx_nic_init_eventq(), it is completely pointless
to do it here.

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