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 959056982a9b ("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: 959056982a9b ("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>