net: phy: dp83867: Explicitly check against sgmii

dp83867 driver only supports sgmii and not all the newer protocols.
Drop the usage of the generic phy_interface_is_sgmii function and just
matchup to the specific mode supported.

Reported-by: Tom Rini <trini@konsulko.com>
Suggested-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Suggested-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/all/b82ac325-4818-8e72-054b-640268dbf806@mailbox.org/


Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>