When loading the brmcfmac module during Linux boot the message "brcmfmac: brcmf_sdio_drivestrengthinit: No SDIO Drive strength init done for chip 4339 rev 2 pmurev 23" occurs quite frequently. Is that a real error?

This is not an error message. The Sterling-LWB doesn't require a drive-strength adjustment command. The message is totally innocent.  All it is really saying is "The chip you're using doesn't need this adjustment, so we're not doing one." It's also notable that in more recent kernels, this was changed to a debug message:  https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/…