bluetooth/hci_bcsp: fix bitrev Kconfig

Fix bluetooth hci_bcsp Kconfig to avoid build errors:

drivers/built-in.o: In function `bcsp_prepare_pkt':
hci_bcsp.c:(.text+0x7e9ac): undefined reference to `bitrev16'
drivers/built-in.o: In function `bcsp_recv':
hci_bcsp.c:(.text+0x7f276): undefined reference to `bitrev16'
hci_bcsp.c:(.text+0x7f293): undefined reference to `bitrev16'
make[1]: *** [.tmp_vmlinux1] Error 1

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Ackey-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>