drivers: ethernet: phy: KSZ8081 PHY Driver improvements

Added changes required for nxp_enet ethernet driver to work
with multiple PHYs and fixed few problems:

- The cfg_link API resets PHY before configuring link. It was moved
  here so the ethernet driver does not have to reset it - not all
  PHYs need reset before configuring link and moving the reset code
  here makes possible to have the reset done in a PHY specific way
  (for example to reset by toggling GPIO pin). It also avoids ethernet
  driver touching PHY registers without locking.
- When reset GPIO is not defined, reset is performed by setting reset
  bit in control register.
- The cfg_link API does not return error when autonegotiation fails.
  This fixes situation when the link is down at system start - ethernet
  driver then skipped setting link-change callback and link was never
  to be detected again.
- Added reset of excessive bits 16-31 when reading register values.
  As only 16 bits are read from PHY, but the API is supposed to read
  into uint32_t, the remaining bits contained previous data after
  a successful read.
- Fixed missing mutex unlock when querying link state and link was down.
- Added missing initializer to link state variables. This could result
  in link state change detection while link was still down, because
  the speed/duplex settings could be random and old and new state
  could be wrongly detected as different.
- Not logging link speed/duplex status when link is not up.

Signed-off-by: Stanislav Poboril <stanislav.poboril@nxp.com>