crypto: hisilicon/qm - set NULL to qm->debug.qm_diff_regs

When the initialization of qm->debug.acc_diff_reg fails,
the probe process does not exit. However, after qm->debug.qm_diff_regs is
freed, it is not set to NULL. This can lead to a double free when the
remove process attempts to free it again. Therefore, qm->debug.qm_diff_regs
should be set to NULL after it is freed.

Fixes: 8be09133 ("crypto: hisilicon/debugfs - Fix debugfs uninit process issue")
Signed-off-by: Chenghai Huang <huangchenghai2@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>