Bluetooth: Controller: Deprioritize adv report buffer allocation

Advertising reports are the only HCI events which we can drop if we
are low on buffers. Allocate them therefore with K_NO_WAIT rather than
K_FOREVER.

Change-Id: I0b7c92647f9be54b8746da837037725f8161a452
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>