uORB: Fix the remaining race conditions from callback unregistration #1164

Job	Run time
variables	1s
fc_matrix (saluki-pi_amp) / build	6m 18s
fc_matrix (saluki-pi_custom_keys, Tools/saluki-sec-scripts/custom_keys/saluki-pi/px4_bin_ed25519_... / build	6m 7s
fc_matrix (saluki-pi_default) / build	6m 42s
fc_matrix (pixhawk) / build	6m 8s
fc_matrix (saluki-v2_amp) / build	6m 21s
fc_matrix (saluki-pi_protected) / build	7m 28s
fc_matrix (saluki-v2_custom_keys, Tools/saluki-sec-scripts/custom_keys/saluki-v2/px4_bin_ed25519_... / build	6m 7s
fc_matrix (saluki-v2_default) / build	6m 16s
fc_matrix (saluki-v2_kernel) / build	9m 13s
fc_matrix (saluki-v3_amp) / build	6m 50s
fc_matrix (saluki-v3_custom_keys, Tools/saluki-sec-scripts/custom_keys/saluki-v3/px4_bin_ed25519_... / build	6m 38s
fc_matrix (saluki-v2_protected) / build	7m 15s
fc_matrix (saluki-v3_default) / build	7m 0s
build px4fwupdater	2m 5s
upload builds to artifactory	1m 21s
upload builds to UAE artifactory	0s
upload px4fwupdater to docker registry	42s
upload px4fwupdater to UAE docker registry	0s
	1h 32m 32s

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