staging: rtl8188eu: make struct field of mlme_handler const

The strings in the uses of struct mlme_handler are constant.
Change the type of field str from 'char *' to 'const char *'.

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