kfifo: fix sparse complaints

This patch fix complaints by the sparse tool when using kfifo_put() with
non scalar types like structures (i.e.
drivers/iio/industrialio-event.c).

Casting a pointer to the value and read this pointer instead of directly
casting the value will fix this.

The generated code is equal.

Signed-off-by: Stefani Seibold <stefani@seibold.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>