Staging: rtl8712: rtl8712_xmit: Use !x instead of x == NULL

Use !x instead of x == NULL. This patch was found by checkpatch.

Signed-off-by: Sandhya Bankar <bankarsandhya512@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>