rtl8192su: make wireless-testing.git a requirement

Charles Moulliard (and Shorrer) have reported an issue,
when they were trying to build rtl8192su [0].

The issue is that the new rtl8192su driver is using
the rtlwifi framework. Therefore the project has to
import any API changes from the wireless-testing
tree and this can cause api-related problems.

[0] <https://github.com/chunkeey/rtl8192su/issues/2>