Staging: rtl8723bs: Use !x instead of NULL comparison

If "x" is compared to NULL, use "!x" instead of it, so as
to follow the kernel coding style.

Signed-off-by: Shreeya Patel <shreeya.patel23498@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>