Fix build errors when RADIO_TEA575X=y, VIDEO_BT848=m, and VIDEO_DEV=m.

The build errors occur due to [in drivers/media/Makefile]:
obj-$(CONFIG_VIDEO_DEV) += radio/
so the (would be) builtin tea575x.o is not being built.

This is also due to drivers/media/radio/Kconfig declaring a bool
Kconfig symbol (RADIO_ADAPTERS) that depends on a tristate (VIDEO_DEV),
so when VIDEO_DEV=m, RADIO_ADAPTERS becomes =y, and then the drivers
that depend on RADIO_ADPATERS can be configured as builtin (=y) or
as loadable modules (=m).

Fix this by converting RADIO_ADAPTERS to a tristate symbol instead
of a bool symbol.

Fixes these build errors:

ERROR: modpost: "snd_tea575x_hw_init" [drivers/media/pci/bt8xx/bttv.ko] undefined!
ERROR: modpost: "snd_tea575x_set_freq" [drivers/media/pci/bt8xx/bttv.ko] undefined!
ERROR: modpost: "snd_tea575x_s_hw_freq_seek" [drivers/media/pci/bt8xx/bttv.ko] undefined!
ERROR: modpost: "snd_tea575x_enum_freq_bands" [drivers/media/pci/bt8xx/bttv.ko] undefined!
ERROR: modpost: "snd_tea575x_g_tuner" [drivers/media/pci/bt8xx/bttv.ko] undefined!

Link: lore.kernel.org/r/202204191711.IKJJFjgU-lkp@intel.com

Fixes: 9958d30f ("media: Kconfig: cleanup VIDEO_DEV dependencies")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>