brcmfmac: fix uninitialized warning

Building brcmfmac for sparc64 gave the following warning:

  CC [M]  drivers/net/wireless/brcm80211/brcmfmac/bcmsdh_sdmmc.o
    bcmsdh_sdmmc.c: In function 'brcmf_sdioh_request_byte':
     bcmsdh_sdmmc.c:89:6: warning: 'err_ret' may be used uninitialized
                          in this function [-Wuninitialized]

Inspecting the code it indeed had a path of execution in
which the return value was used uninitialized. This patch
fixes that code path.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>