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 <[email protected]>

Author: Jordan Yates

drivers/bluetooth/hci/spi.c

@@ -166,9 +166,8 @@ static int bt_spi_get_header(uint8_t op, uint16_t *size)
 			break;
 		}
 		if (reading) {
-			/* When reading, keep looping if read buffer is not valid */
-			loop_cond = ((header_slave[STATUS_HEADER_TOREAD] == 0U) ||
-					(header_slave[STATUS_HEADER_TOREAD] == 0xFF));
+			/* When reading, keep looping if there is not yet any data */
+			loop_cond = header_slave[STATUS_HEADER_TOREAD] == 0U;
 		} else {
 			/* When writing, keep looping if all bytes are zero */
 			loop_cond = ((header_slave[1] | header_slave[2] | header_slave[3] |