rndis_wlan: fix checking for default value

Thresholds uses -1 to indicate that default value should be used.
Since thresholds are unsigned sign checking makes no sense.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576



Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>