net: phy: Make phy_interface_is_rgmii a switch statement

Recent commit 75d28899 ("net: phy: Synchronize PHY interface modes
with Linux") reordered the enum definitions. This exposed a problem
in range checking functions to identify the interface type. Though
this specific api wasn't impacted (all the RGMII definitions remained
within range), this experience should be used to never to have to face
this kind of challenge again.

While it is possible for the phy drivers to use the enums directly,
drivers such as dp83867, dp83869, marvell, micrel_ksz90x1 etc use this
api.

Reported-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>