staging: rtl8188eu: remove set but unused variable from rtw_get_sec_ie

staging: rtl8188eu: remove set but unused variable from rtw_get_sec_ie

Remove set but unused variable 'sec_idx' from the rtw_get_sec_ie
function inside core/rtw_ieee80211.c, to fix a kernel test robot warning
introduced by recent removal of erroneous debug statements.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210625183007.7065-1-phil@philpotter.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>