V4L/DVB (7848): Fix dependencies for tuner-xc2028 and em28xx-dvb

em28xx-dvb doesn't need FW_LOADER. Instead, tuner-xc2028 needs to select
FW_LOADER.

Also, this can happen only if HOTPLUG is selected, since FW_LOADER
is dependent on HOTPLUG.

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