[media] pulse8-cec: declare function as static

drivers/staging/media/pulse8-cec/pulse8-cec.c:104:6: warning: no previous prototype for 'pulse8_irq_work_handler' [-Wmissing-prototypes]
 void pulse8_irq_work_handler(struct work_struct *work)
      ^~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>