bt_power: Addressed NULL pointer dereferencing in power module

- On some platforms, we do not specify any regulator
  for forced chip power down(bt_chip_pwd), So bt_chip_pwd
  will be NULL in such cases.

- Added a NULL check before dereferencing bt_chip_pwd
  in power module

CRs-Fixed: 2269588
Change-Id: Iee0da7d643ad0254f043a33620e9b5c631a68480
Signed-off-by: Ramesh Bandari <ramban@codeaurora.org>