firmware: tegra: Switch over to memdup_user()

This patch fixes the following Coccinelle warning:

drivers/firmware/tegra/bpmp-debugfs.c:379: WARNING opportunity for memdup_user

Use memdup_user() rather than duplicating its implementation. This is a
little bit restricted to reduce false positives.

Signed-off-by: Qing Wang <wangqing@vivo.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>