rndis_wlan: tighten check of rndis_query_oid return

clang static analysis reports this problem

rndis_wlan.c:3147:25: warning: Assigned value is garbage or undefined
                wiphy->max_num_pmkids = le32_to_cpu(caps.num_pmkids);
                                      ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The setting of caps happens here, with a call to rndis_query_oid()

	retval = rndis_query_oid(usbdev,
	if (retval >= 0) {

Reviewing rndis_query_oid() shows that on success 0 is returned,
failure is otherwise.  So the retval check is not tight enough.
So tighten the retval check.  Similar problem in
rndis_wlan_get_caps().

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200811140219.8412-1-trix@redhat.com