staging: wilc1000: rename u32duration in wilc_remain_on_channel

This patch renames u32duration to duration to remove u32 prefix.
There is no need u32 prefix to show data type of this variable.

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