firmware: qemu_fw_cfg.c: fix typo FW_CFG_DATA_OFF

s/FW_CTRL_DATA_OFF/FW_CFG_DATA_OFF/

Signed-off-by: Valentin Rothberg <valentin.rothberg@posteo.net>
Signed-off-by: Andreas Ziegler <andreas.ziegler@fau.de>
Acked-by: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>