Bluetooth: Take advantage of net_buf_get_timeout()

Use the more explicit net_buf_get_timeout() call where it makes sense
(e.g. where we always want to wait or never want to wait).

Change-Id: Id1eabe0ad2f9fa79f7be39e51fefa5161f9cc550
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>