net: designware: drop compatible altr, socfpga-stmmac

The same compatible = "altr,socfpga-stmmac" appears in both
drivers/net/designware.c and drivers/net/dwmac_socfgpa.c,
creating ambiguity in which driver will be bound.

For Intel/Altera SoC devices, dwmac_socfpga.c is the correct driver.
So drop the compatible string from designware.c.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>