Staging: rtl8192su: Build should select WIRELESS_EXT

The prevailing idiom is to select WIRELESS_EXT not depend on it.

Depending leaves this driver in a situation where it can only
be built if another driver that selects WIRELESS_EXT has been enabled.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>