sfc: mark some unexported symbols as static

efx_default_channel_want_txqs() is only used in efx.c, while
 efx_ptp_want_txqs() and efx_ptp_channel_type (a struct) are only used
 in ptp.c.  In all cases these symbols should be static.

Fixes: 2935e3c3

 ("sfc: on 8000 series use TX queues for TX timestamps")
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
[ecree@solarflare.com: rewrote commit message]
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>