altera_tse: m88e1111s: Honor device flags regarding PHY interface mode

Note: This is kind of guess work. The current code is preserved for
all RGMII related modes. It is different for flags=0 (GMII) and flags=5
(SGMII). The last case, SGMII, is successfully tested on
Altera's Terasic DE4.

Signed-off-by: Joachim Foerster <joachim.foerster@missinglinkelectronics.com>