wimax/i2400m: clean up & add a payload argument to i2400m_schedule_work()

Forthcoming commits use having a payload argument added to
i2400m_schedule_work(), which then becomes nearly identical to
i2400m_queue_work().

This patch thus cleans up both's implementation, making it share
common helpers and adding the payload argument to
i2400m_schedule_work().

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>