[media] pulse8-cec: fix compiler warning

pulse8-cec.c: In function 'pulse8_connect':
pulse8-cec.c:447:2: warning: 'pa' may be used uninitialized in this function [-Wmaybe-uninitialized]
  cec_s_phys_addr(pulse8->adap, pa, false);
  ^
pulse8-cec.c:609:6: note: 'pa' was declared here
  u16 pa;
      ^

As far as I can tell, this can't actually happen. Still, it is better to just
initialize it.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>