firmware: qemu_fw_cfg: constify 'struct bin_attribute'

firmware: qemu_fw_cfg: constify 'struct bin_attribute'

The sysfs core now allows instances of 'struct bin_attribute' to be
moved into read-only memory. Make use of that to protect them against
accidental or malicious modifications.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Gabriel Somlo <somlo@cmu.edu>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/20250114-sysfs-const-bin_attr-qemu_fw_cfg-v1-1-76f525a3ee72@weissschuh.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>