"bu21013_data" and "struct bu21013_ts_data" are a tad long, let's call them
"ts" and "struct bu21013_ts".
Also rename retval to error in bu21013_init_chip() and adjust formatting;
i2c_smbus_write_byte_data() returns negative on error and 0 on success, so
we simply test if whether erro is 0 or not.