samples: bluetooth: hci_spi: IRQ assert delay

Enfore a minimum duration between edges on the IRQ pin to give the HCI
controller time to detect the state change. Without this delay and with
several packets queued on the `rx_queue`, the pin could be re-asserted
almost immediately after being cleared.

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