nl80211: fix OBSS PD min and max offset validation

nl80211: fix OBSS PD min and max offset validation

The SRG min and max offset won't present when SRG Information Present of
SR control field of Spatial Reuse Parameter Set element set to 0. Per
spec. IEEE802.11ax D7.0, SRG OBSS PD Min Offset ≤ SRG OBSS PD Max
Offset. Hence fix the constrain check to allow same values in both
offset and also call appropriate nla_get function to read the values.

Fixes: f4509e6c4e61 ("cfg80211: add support for parsing OBBS_PD attributes")
Signed-off-by: Rajkumar Manoharan <rmanohar@codeaurora.org>
Link: https://lore.kernel.org/r/1601278091-20313-1-git-send-email-rmanohar@codeaurora.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>