Replace usage of HCI_GET_CMD_BUF() with osi_malloc()

* Remove HCI_GET_CMD_BUF() wrapper macro, and replace it with
  its default implementation: (BT_HDR *)osi_malloc(HCI_CMD_BUF_SIZE)

* Remove explicit checks for NULL when HCI_GET_CMD_BUF() was used.

Bug: 27048759
Change-Id: I7c71b408f40d58f6ebf3a310aa806daba3342f77