drm/amdgpu: fix eGPU hotplug regression

The driver needs to wait for the on board firmware
to finish its initialization before probing the card.
Commit 95905698 ("drm/amdgpu: Fix discovery initialization failure during pci rescan")
switched from using msleep() to using usleep_range() which
seems to have caused init failures on some navi1x boards. Switch
back to msleep().

Fixes: 95905698 ("drm/amdgpu: Fix discovery initialization failure during pci rescan")
Closes: drm/amd#3559
Closes: drm/amd#3500


Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Ma Jun <Jun.Ma2@amd.com>
(cherry picked from commit c69b07f7)
Cc: stable@vger.kernel.org # 6.10.x