drivers: ethernet: ksz8081: simplify gpio reset logic in `reset`

Simplifies GPIO reset logic in `phy_mc_ksz8081_reset()` by introducing
a dedicated function, `phy_ksz8081_reset_gpio`. If this function returns
`-ENODEV`, it will fall back to using a command-based reset instead.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>