liteeth/gen: Use hw_init_reset for rgmii

[mkj]

I'm not sure of the original reason for this being marked FIXME, so if the hw_init_reset needs to remain I can instead make this a configuration option for litedram_gen.

This re-enables hw_init_reset that was disabled in ea24ff6 ("liteeth_gen: improve readability and add clk_freq checks.")

A chip reset is necessary using the Microchip KSZ9031RNX PHY on an antmicro-artix-dc-scm board (possibly depending on startup power timing)

[enjoy-digital]

Hi @mkj,

sorry for the delay. The FIXME was probably here when used with the HW stack, I'll have a closer look at the use-case that was requiring it.