staging/rtl81*: build as loadable modules only

These 3 drivers contain much duplicated (triplicated) code, so
building them as built-in results in many errors like:

(.text+0x1b160): multiple definition of `ieee80211_sta_ps_sleep'

Prevent this configuration by making them all buildable only as
loadable modules (similar to the vt665[56] patch last week).

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>