Staging: rtl8188eu: ioctl_linux: Clean up if function returned NULL on failure.

Clean some all functions to return NULL on failure.
Used !x instead of x==NULL.

Signed-off-by: Amit Ghadge <amitg.b14@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>