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

drivers/staging/rtl8187se/Kconfig

 config R8187SE
 	tristate "RealTek RTL8187SE Wireless LAN NIC driver"
 	depends on PCI && WLAN
+	depends on m
 	select WIRELESS_EXT
 	select WEXT_PRIV
 	select EEPROM_93CX6

drivers/staging/rtl8192e/Kconfig

 config RTL8192E
 	tristate "RealTek RTL8192E Wireless LAN NIC driver"
 	depends on PCI && WLAN
+	depends on m
 	select WIRELESS_EXT
 	select WEXT_PRIV
 	select CRYPTO

drivers/staging/rtl8192u/Kconfig

 config RTL8192U
 	tristate "RealTek RTL8192U Wireless LAN NIC driver"
 	depends on PCI && WLAN && USB
+	depends on m
 	select WIRELESS_EXT
 	select WEXT_PRIV
 	select CRYPTO