fw_cfg: fix sparse warnings with fw_cfg_file

Modify fw_cfg_sysfs_entry to store entry values, instead of reusing
the restricted types.

Fixes warnings such as:

$ make C=1 CF=-D__CHECK_ENDIAN__ drivers/firmware/qemu_fw_cfg.o

drivers/firmware/qemu_fw_cfg.c:491:29: warning: incorrect type in assignment (different base types)
drivers/firmware/qemu_fw_cfg.c:491:29:    expected restricted __be32 [usertype] size
drivers/firmware/qemu_fw_cfg.c:491:29:    got unsigned int
drivers/firmware/qemu_fw_cfg.c:492:31: warning: incorrect type in assignment (different base types)
drivers/firmware/qemu_fw_cfg.c:492:31:    expected restricted __be16 [usertype] select
drivers/firmware/qemu_fw_cfg.c:492:31:    got int

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>