firmware_loader: fix up to_fw_sysfs() to preserve const

to_fw_sysfs() was changed in commit 23680f0b ("driver core: make
struct class.dev_uevent() take a const *") to pass in a const pointer
but not pass it back out to handle some changes in the driver core.
That isn't the best idea as it could cause problems if used incorrectly,
so switch to use the container_of_const() macro instead which will
preserve the const status of the pointer and enforce it by the compiler.

Fixes: 23680f0b

 ("driver core: make struct class.dev_uevent() take a const *")
Cc: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Russ Weight <russell.h.weight@intel.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20221205121206.166576-4-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>