net: stmmac: dwmac-stm32: Clean up the debug prints

Use dev_err()/dev_dbg() and phy_modes() to print PHY mode instead of
pr_debug() and hand-written PHY mode decoding. This way, each debug
print has associated device with it and duplicated mode decoding is
removed.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Christophe Roullier <christophe.roullier@foss.st.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>