staging: rtl8723bs: core: Change NULL comparisons to Boolean negation

Change NULL comparisons to Boolean negations. Issue found with
Coccinelle using matchnull.cocci.

Signed-off-by: Nishka Dasgupta <nishka.dasgupta@yahoo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>