bluetooth: hci: spi: extract message handling to func

Extract the HCI message handling out to a dedicated function to simplify
the main thread function. This also solves a bug as a side effect.

Previously `discardable` and `timeout` were never being reset after
an advertising report was received, resulting in ALL events after the
first advertising report being treated as discardable.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>