sfc/siena: Rename peripheral functions to avoid conflicts with sfc

For siena use efx_siena_ as the function prefix.
This patch covers selftest.h, ptp.h, net_driver.h and ethtool_common.h.
efx_ethtool_fill_self_tests() can become static.
Some functions in ptp.c can also become static.
Rename loopback_mode in net_driver.h.

Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>