bluetooth: hci_bcsp.c small cleanups/api users

Use bitrev16 from bitrev.h

Use the get_unaligned_be16 to get the crc from the packet, create a small helper function for this.

Fix a shadowed variable sparse warning:
drivers/bluetooth/hci_bcsp.c:218:26: warning: symbol 'hdr' shadows an earlier one drivers/bluetooth/hci_bcsp.c:187:5: originally declared here

---
 drivers/bluetooth/Kconfig    |    1 +
 drivers/bluetooth/hci_bcsp.c |   44 ++++++++++++-----------------------------
 2 files changed, 14 insertions(+), 31 deletions(-)

index 075598e..a235ca7 100644
@@ -71,6 +71,7 @@ config BT_HCIUART_H4
 config BT_HCIUART_BCSP
 	bool "BCSP protocol support"
 	depends on BT_HCIUART
+	select BITREVERSE
 	help
 	  BCSP (BlueCore Serial Protocol) is serial protocol for communication 
 	  between Bluetooth device and host. This protocol is required for non

1.5.6.rc1.257.gba91d