staging: wilc1000: rename u32duration in struct remain_ch

The patch renames u32duration to duration that is a member of struct
remain_ch. The prefix u32 shows data type of its member, but there is no
need to use u32 prefix to represent data type.

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>