mwifiex: Use to_delayed_work()

Introduce the use of to_delayed_work() helper function instead of open
coding it with container_of()

A simplified version of the Coccinelle semantic patch used to make
this change is:

//<smpl>
@@
expression a;
symbol work;
@@
- container_of(a, struct delayed_work, work)
+ to_delayed_work(a)
//</smpl>

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>