net: phy: fix yt8511 clang uninitialized variable warning

clang doesn't preinitialize variables. If phy_select_page failed and
returned an error, phy_restore_page would be called with `ret` being
uninitialized.
Even though phy_restore_page won't use `ret` in this scenario,
initialize `ret` to silence the warning.

Fixes: 48e8c6f1

 ("net: phy: add driver for Motorcomm yt8511 phy")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>