staging/Makefile: Don't compile a media driver there

staging/Makefile: Don't compile a media driver there

Otherwise, with driver as "y", compilation will break:

drivers/staging/media/as102/built-in.o: In function `as10x_cmd_get_demod_stats':
/home/v4l/work_trees/linux-next/drivers/staging/media/as102/as10x_cmd.c:306: multiple definition of `as10x_cmd_get_demod_stats'
drivers/staging/media/built-in.o:/home/v4l/work_trees/linux-next/drivers/staging/media/as102/as10x_cmd.c:306: first defined here
drivers/staging/media/as102/built-in.o:(.data+0x88): multiple definition of `as102_st_fw1'
drivers/staging/media/built-in.o:(.data+0x88): first defined here

as the same driver is already at staging/media/Makefile.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>