Staging: rtl8188eu: core: rtw_mlme.c: Replace NULL comparision.

Replace NULL comparison with '!' in the file rtw_mlme.c
Issue found by checkpatch.pl semantic patch results for rtw_mlme.c

CHECK:Comparision to NULL could be written "!candidate"

Signed-off-by: Sanjana Sanikommu <sanjana99reddy99@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>