Bluetooth: btusb: Remove return statement in btintel_reset_to_bootloader

When building with Clang and CONFIG_BT_INTEL unset, the following error
occurs:

In file included from drivers/bluetooth/hci_ldisc.c:34:
drivers/bluetooth/btintel.h:188:2: error: void function
'btintel_reset_to_bootloader' should not return a value [-Wreturn-type]
        return -EOPNOTSUPP;
        ^      ~~~~~~~~~~~
1 error generated.

Remove the unneeded return statement to fix this.

Fixes: b9a2562f

 ("Bluetooth: btusb: Trigger Intel FW download error recovery")
Link: https://github.com/ClangBuiltLinux/linux/issues/743
Reported-by:  <ci_notify@linaro.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>