V4L/DVB (7849): cx88: fix Kconfig depencencies for FW_LOADER

cx88 doesn't need support for FW_LOADER. Instead, this is required only for
cx88-blackbird.

Also, cx88-blackbird depends on HOTPLUG, due to FW_LOADER dependency.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>