brcmfmac: fix false-positive -Wmaybe-unintialized warning

When CONFIG_NO_AUTO_INLINE is set, we get a false-postive warning
for the brcmf_fw_request_nvram_done() function, after gcc figures
out that brcmf_fw_nvram_from_efi() might not set the 'data_len'
variable, but fails to notice that it always returns NULL:

drivers/net/wireless/broadcom/brcm80211/brcmfmac/firmware.c: In function 'brcmf_fw_request_nvram_done':
drivers/net/wireless/broadcom/brcm80211/brcmfmac/firmware.c:560:11: error: 'data_len' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Mark it 'inline' to force gcc to understand this.

Fixes: ce2e6db5 ("brcmfmac: Add support for getting nvram contents from EFI variables")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>