staging: rtl8192e: replace comparison to NULL by bool

Fixed Comparison to NULL can be written as '!...' by replacing it with
simpler form i.e boolean expression. This makes code more readable alternative.
Reported by checkpatch.

Signed-off-by: Mitali Borkar <mitaliborkar810@gmail.com>
Link: https://lore.kernel.org/r/YHDnWpWztxeZospi@kali


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>