ionic: Report "rx_bits_phy" stat to ethtool

ionic: Report "rx_bits_phy" stat to ethtool

This stat contains the number of total bits that the PHY has received;
it's useful for BER calculations. Add it to the ethtool stats output.

However, since this is one of the new "extra port stats", it's reported
in a different manner than the existing port stats and only
conditionally added to the ethtool stats output list: both the
DEV_CAP_EXTRA_STATS capability must be supported by the firmware, and
the firmware must set the value of the statistic to something other than
IONIC_STAT_INVALID.

To help support this scheme, the extra port stats region is initialized to
0xff's/IONIC_STAT_INVALID by the driver, to ensure the statistics that
the driver knows about but the firmware does not are still invalid
to the driver.

Signed-off-by: Eric Joyner <eric.joyner@amd.com>
Link: https://patch.msgid.link/20260614205303.48088-4-eric.joyner@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>