HSI: hsi_char: Remove max_data_size from sysfs

Remove max_data_size sysfs entry. Otherwise is possible
to have a buffer overrun if its value is increased after
the device is open.

Signed-off-by: Carlos Chinea <carlos.chinea@nokia.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>