drm/amdgpu: fix eGPU hotplug regression

[ Upstream commit 9cead81eff635e3b3cbce51b40228f3bdc6f2b8c ]

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: https://gitlab.freedesktop.org/drm/amd/-/issues/3559
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/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 c69b07f7bbc905022491c45097923d3487479529)
Cc: stable@vger.kernel.org # 6.10.x
Signed-off-by: Sasha Levin <sashal@kernel.org>