libsframe: rename sframe_fre_* internal APIs to use data word instead of offset

Rename three internal functions:
  - sframe_fre_get_offset_count to sframe_fre_get_dataword_count
  - sframe_fre_get_offset_size to sframe_fre_get_dataword_size
  - sframe_fre_offset_bytes_size to sframe_fre_datawords_bytes_size.

libsframe/
	* sframe.c: Rename functions and variables.