net: usb: dm9601: fix wrong return value in dm9601_mdio_read

The MII code does not check the return value of mdio_read (among
others), and therefore no error code should be sent. A previous fix to
the use of an uninitialized variable propagates negative error codes,
that might lead to wrong operations by the MII library.

An example of such issues is the use of mii_nway_restart by the dm9601
driver. The mii_nway_restart function does not check the value returned
by mdio_read, which in this case might be a negative number which could
contain the exact bit the function checks (BMCR_ANENABLE = 0x1000).

Return zero in case of error, as it is common practice in users of
mdio_read to avoid wrong uses of the return value.

Fixes: 8f8abb86

 ("net: usb: dm9601: fix uninitialized variable use in dm9601_mdio_read")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Peter Korsgaard <peter@korsgaard.com>
Link: https://lore.kernel.org/r/20240225-dm9601_ret_err-v1-1-02c1d959ea59@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>