regmap: mdio: Don't modify output if error happened

regmap_mdio_read() breaks the principle of "no touch output till it's known
that the operation succeeds". Refactor it accordingly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210520120518.30490-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>