staging: unisys: remove typedef for PERIODIC_WORK

In periodic_work.h, remove the typedef PERIODIC_WORK and replace it with
struct periodic_work, and update functions used to manipulate the
structure to use the new name. The functions used to manipulate the
PERIODIC_WORK type are updated to use the fixed name, corrected for
clarity, and changed to not use periodic_work as the parameter's name.

Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>