Bluetooth: monitor: Drop data instead of holding IRQ lock for long

The IRQ lock should never be held for long periods of time. Instead of
using the IRQ lock to prevent monitor protocol corruption simply drop
the data. Follow-up patches will add additional drop count tracking so
that these get reported properly over the protocol.

Change-Id: If498125b29f1b58bed676c78ad2062e2aa206318
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>