PM: hibernate: Do not initialize error in swap_write_page()

'error' first receives the function result before it is used, and it
does not need to be assigned a value during definition.

Signed-off-by: Li zeming <zeming@nfschina.com>
[ rjw: Subject rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>