wifi: brcmfmac: fix up incorrect 4.14.y backport for brcmf_fw_map_chip_to_name()

In commit bc45aa19 which is commit
81d17f6f upstream, the return value of the
error condition needs to be reworked to return a real error and not NULL.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Cc: Dokyung Song <dokyungs@yonsei.ac.kr>
Cc: Jisoo Jang <jisoo.jang@yonsei.ac.kr>
Cc: Minsuk Kang <linuxlovemin@yonsei.ac.kr>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: Sasha Levin <sashal@kernel.org>
Link: https://lore.kernel.org/r/Y8gccXXyE30sbPSg@dev-arch.thelio-3990X


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>