drivers: ethernet: xlnx_gem: fix PHY_MRVL_MODE_CONFIG_MASK

According to the datasheet for Marvell PHY [1], the mode field of the
General Control Register 1 Page 18 occupies bits [2:0]. However, the macro
PHY_MRVL_MODE_CONFIG_MASK specifies the mask as 0x3, which would
correspond only to [1:0]. The code in phy_xlnx_gem_marvell_alaska_cfg()
uses the mask to set the mode field to 0 to set "RGMII (System mode) to
Copper" mode. Unfortunately, different chips have different reset values
(111 or 000) and in first case, the code would set the field to 100,
instead of 000.

Without this change, ethernet on Avnet MicroZed (Marvel Alaska 88E1512 PHY)
does not work.

Signed-off-by: Marek Vedral <vedrama5@fel.cvut.cz>

[1]: https://www.marvell.com/content/dam/marvell/en/public-collateral/phys-transceivers/marvell-ethernet-phys-alaska-88e151x-datasheet.pdf