brcmfmac: avoid gcc-5.1 warning

gcc-5.0 gained a new warning in the fwsignal portion of the brcmfmac
driver:

drivers/net/wireless/brcm80211/brcmfmac/fwsignal.c: In function 'brcmf_fws_txs_process':
drivers/net/wireless/brcm80211/brcmfmac/fwsignal.c:1478:8: warning: 'skb' may be used uninitialized in this function [-Wmaybe-uninitialized]

This is a false positive, and marking the brcmf_fws_hanger_poppkt function
as 'static inline' makes the warning go away. I have checked the object
file output and while a little code gets moved around, the size of
the binary remains identical.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>