wifi: ath11k: Defer on rproc_get failure

If we already have gotten the rproc_handle (meaning the "qcom,rproc"
property is defined in the devicetree), it's a valid state that the
remoteproc module hasn't probed yet so we should defer probing instead
of just failing to probe.

This resolves a race condition when the ath11k driver probes and fails
before the wpss remoteproc driver has probed, like the following:

  [    6.232360] ath11k 17a10040.wifi: failed to get rproc
  [    6.232366] ath11k 17a10040.wifi: failed to get rproc: -22
  [    6.232478] ath11k: probe of 17a10040.wifi failed with error -22
       ...
  [    6.252415] remoteproc remoteproc2: 8a00000.remoteproc is available
  [    6.252776] remoteproc remoteproc2: powering up 8a00000.remoteproc
  [    6.252781] remoteproc remoteproc2: Booting fw image qcom/qcm6490/fairphone5/wpss.mdt, size 7188

So, defer the probe if we hit that so we can retry later once the wpss
remoteproc is available.

Tested-on: WCN6750 hw1.0 AHB WLAN.MSL.1.0.1-01264-QCAMSLSWPLZ-1.37886.3

Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20231027-ath11k-rproc-defer-v1-1-f6b6a812cd18@fairphone.com

drivers/net/wireless/ath/ath11k/ahb.c

@@ -803,8 +803,8 @@ static int ath11k_core_get_rproc(struct ath11k_base *ab)
 
 	prproc = rproc_get_by_phandle(rproc_phandle);
 	if (!prproc) {
-		ath11k_err(ab, "failed to get rproc\n");
-		return -EINVAL;
+		ath11k_dbg(ab, ATH11K_DBG_AHB, "failed to get rproc, deferring\n");
+		return -EPROBE_DEFER;
 	}
 	ab_ahb->tgt_rproc = prproc;