staging: rtl8192e: Rectify pointer comparisions with NULL

This patch simplifies code by replacing explicit NULL comparison
with ! or unmark operator
Reported by checkpatch.pl for comparison to NULL could be
written '!foo' or 'foo'

Signed-off-by: Suniel Mahesh <suniel.spartan@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>