Improve interface of FlexRAN RAN API: Return protobuf types

The functions of the RAN API should not return the types of OAI, but protobuf
types. Previously, they would return OAI types and the calling code "marshals"
this into Protobuf type. This commit changes the following functions and
modifies the calling code to directly store the protobuf type:
* flexran_get_hopping_mode()
* flexran_get_phich_resource()
* flexran_get_phich_duration()
* flexran_get_ul_cyclic_prefix_length()
* flexran_get_dl_cyclic_prefix_length()
* flexran_get_duplex_mode() usage
* flexran_get_enable64QAM()