gnss: change of_property_read to device_property_read

change of_property_read_u32 for the current-speed property
to use the device_property_read_u32 offered by the
Unified Device Properties Interface, this helps
passing the properties over a suitably populated struct
property_entry.

Signed-off-by: Vaishnav M A <vaishnav@beagleboard.org>