usbatm: fix potential NULL pointer dereference

The dereference to 'instance' in the debug code should be moved
below the NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>