wlcore: add const to bin_attribute structure

Add const to bin_attribute structure as it is only passed to the
functions sysfs_{remove/create}_bin_file. The corresponding arguments
are of type const, so declare the structure to be const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>