staging: rtl8192e: rtllib_crypt_tkip: rewritten comparison to NULL

Rewritten a comparison to NULL with a negation operator in
accordance with the Linux kernel coding-style regulations. The fix
was directly recommended by the checkpatch script.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Signed-off-by: Aaron Lawrence <t4rmin@zohomail.com>
Link: https://lore.kernel.org/r/ca33296630627020694f4b653580f689a8a3d1c7.1666787061.git.t4rmin@zohomail.com


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