device property: constify fwnode_get_phy_mode() argument

fwnode_get_phy_mode() does not modify the fwnode argument, merely
using it to obtain the phy-mode property value. Therefore, it can
be made const.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/E1pfdh9-00EQ8t-HB@rmk-PC.armlinux.org.uk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>