Bluetooth: try to improve CONFIG_SERIAL_DEV_BUS dependency

With CONFIG_SERIAL_DEV_BUS=m, the hci_serdev.o file does not actually
get built into hci_uart.o as the Makefile doesn't pick it up, leading
to a link error with anything referring to it:

ERROR: "hci_uart_register_device" [drivers/bluetooth/hci_nokia.ko] undefined!
scripts/Makefile.modpost:91: recipe for target '__modpost' failed

Changing this in the Makefile would cause another problem when
hci_uart itself is built-in and cannot reference symbols from the
serdev module.

This tries to address both problems by introducing a new hidden
Kconfig symbol that controls both the compilation of hci_serdev.o
and whether the Nokia driver can be selected. This seems to address
the problem for me, though there might be a better way to do it.

Fixes: 7bb31868 ("Bluetooth: add nokia driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>