net: phy: mv88e61xx: Fix uninitialized variable warning

The variable 'res' may be unused uninitialized if our call to
mv88e61xx_port_read (register read) fails and we goto the error
handling section.  In this case we set 'res' to -EIO to indicate why we
failed.

Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Chris Packham <judge.packham@gmail.com>
Cc: Kevin Smith <kevin.smith@elecsyscorp.com>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Signed-off-by: Tom Rini <trini@konsulko.com>