ieee80211: fix HE SPR size calculation

The he_sr_control field is just a u8, so le32_to_cpu()
shouldn't be applied to it; this was evidently copied
from ieee80211_he_oper_size(). Fix it, and also adjust
the type of the local variable.

Fixes: ef11a931

 ("mac80211: HE: add Spatial Reuse element parsing support")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20200325090918.dfe483b49e06.Ia53622f23b2610a2ae6ea39a199866196fe946c1@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>