libsframe: use uint8_t for return type of sframe_fre_get_base_reg_id

Use a more appropriate data type.

include/
	* sframe-api.h (sframe_fre_get_base_reg_id): Use uint8_t as
	return type.
libsframe/
	* sframe-dump.c (dump_sframe_func_with_fres): Use uint8_t type
	for base reg id.
	* sframe.c (sframe_fre_get_base_reg_id): Use uin8_t as return
	type.