arm: ls102xa: Fix RGMII phy-connection-type FDT fixup

arm: ls102xa: Fix RGMII phy-connection-type FDT fixup

In ft_fixup_enet_phy_connect_type(), use strlen() instead of sizeof()
on the pointer result of phy_string_for_interface(). sizeof() was
returning the size of the pointer (4 bytes), resulting in the
phy-connection-type being set to "rgmi" rather than "rgmii-id".

Signed-off-by: Brendan Shanks <brendan.shanks@teradek.com>
Cc: York Sun <york.sun@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>