We see the following link error with CONFIG_BT_HCIUART=y,
CONFIG_BT_HCIUART_LL=y and CONFIG_SERIAL_DEV_BUS=m:

drivers/built-in.o: In function 'll_close':
supp.c:(.text+0x55add4): undefined reference to 'serdev_device_close'
supp.c:(.text+0x55add4): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol 'serdev_device_close'
drivers/built-in.o: In function 'll_open':
supp.c:(.text+0x55aed0): undefined reference to 'serdev_device_open'
supp.c:(.text+0x55aed0): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol 'serdev_device_open'
drivers/built-in.o: In function `hci_ti_probe':
supp.c:(.text+0x55b00c): undefined reference to 'hci_uart_register_device'
supp.c:(.text+0x55b00c): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol 'hci_uart_register_device'
drivers/built-in.o: In function `ll_setup':
supp.c:(.text+0x55b08c): undefined reference to 'serdev_device_set_flow_control'
supp.c:(.text+0x55b08c): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol 'serdev_device_set_flow_control'
supp.c:(.text+0x55b324): undefined reference to 'serdev_device_set_baudrate'
supp.c:(.text+0x55b324): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol 'serdev_device_set_baudrate'
drivers/built-in.o: In function 'll_init':
supp.c:(.init.text+0x1b508): undefined reference to '__serdev_device_driver_register'
supp.c:(.init.text+0x1b508): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol '__serdev_device_driver_register'

Fix this by dependig BT_HCIUART_LL on the BT_HCIUART_SERDEV symbol.
This implies a dependency on BT_HCIUART and hci_ll.c is only compiled in
if SERIAl_DEV_BUS is built in or SERIAL_DEV_BUS and BT_HCIUART are
modules.

Fixes: 37180552 ("bluetooth: hci_uart: add LL protocol serdev driver support")
Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>