mtd: nand: gpmi: fixup return type of wait_for_completion_timeout

return type of wait_for_completion_timeout is unsigned long not int. The
return variable is renamed to reflect its use and the type adjusted to
unsigned long.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Huang Shijie <shijie.huang@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>