staging: ks7010: avoid dereferencing packet if it is null

Updating tx_bytes from packet->len if packet is null will cause
a null pointer dereference, so only update tx_bytes if it packet
is not null.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>