staging: rtl8188eu: replace while with shorter for loop

Simplify rtw_get_rateset_len() by replacing the while loop
with a shorter for loop. Also replace tabs with spaces in
the definition line.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>