bluetooth: hci: spi: 0xFF is a valid payload length

A payload size of 0xFF is valid and should not be a reason to keep
looping for more headers. The complete reqiurement is now:
```
header_slave[STATUS_HEADER_READY] == READY_NOW &&
header_slave[STATUS_HEADER_TOREAD] > 0
```
This fixes events being dropped when the payload size is 255.

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