brcmfmac: remove duplicate pointer variable from brcmf_sdio_firmware_callback()

In brcmf_sdio_firmware_callback() two pointer variables were used
pointing to the same construct. Get rid of sdiodev variable.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>