firmware: tegra: bpmp: Fix error paths in debugfs

Some error paths in mrq_debug_read and bpmp_debug_show would overwrite
the return error code with a subsequent call to mrq_debug_close.

Change the code to only change the error code if there was no prior
error.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>