Input: bu21013_ts - rename some variables

Input: bu21013_ts - rename some variables

"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.

Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>