net: phy: dp83867: fix get nvmem cell fail

If CONFIG_NVMEM is not set of_nvmem_cell_get, of_nvmem_device_get
functions will return ERR_PTR(-EOPNOTSUPP) and "failed to get nvmem
cell io_impedance_ctrl" error would be reported despite "io_impedance_ctrl"
is completely missing in Device Tree and we should use default values.

Check -EOPNOTSUPP togather with -ENOENT to avoid this situation.

Fixes: 5c2d0a6a

 ("net: phy: dp83867: implement support for io_impedance_ctrl nvmem cell")
Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220805084843.24542-1-nikita.shubin@maquefel.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>