netdevsim: add fw_update_flash_chunk_time_ms debugfs knobs

Netdevsim emulates firmware update and it takes 5 seconds to complete.
For some use cases, this is too long and unnecessary. Allow user to
configure the time by exposing debugfs a knob to set chunk time.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250722091945.79506-1-jiri@resnulli.us


Signed-off-by: Jakub Kicinski <kuba@kernel.org>