mtd: fsmc_nand: match wait_for_completion_timeout return type

return type of wait_for_completion_timeout is unsigned long not int. An
appropriately named unsigned long is added and the assignment fixed up.
This not only should help readability but also handles corner cases
properly.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>