USB: pl2303: cosmetic changes to pl2303_buf_{clear, data_avail}

Changes the functions pl2303_buf_clear and pl2303_buf_data_avail for
the purpose of keeping them under the 80 column limit, making them
more similar to similar functions and making then simpler.

Signed-off-by: Thiago Galesi <thiagogalesi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>