net: usb: sr9700: fix uninitialized variable use in sr_mdio_read

commit 08f3a5c3 upstream.

It could lead to error happen because the variable res is not updated if
the call to sr_share_read_word returns an error. In this particular case
error code was returned and res stayed uninitialized. Same issue also
applies to sr_read_reg.

This can be avoided by checking the return value of sr_share_read_word
and sr_read_reg, and propagating the error if the read operation failed.

Found by code review.

Cc: stable@vger.kernel.org
Fixes: c9b37458

 ("USB2NET : SR9700 : One chip USB 1.1 USB2NET SR9700Device Driver Support")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Shigeru Yoshida <syoshida@redhat.com>
Reviewed-by: Hariprasad Kelam <hkelam@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>