firmware: Fix firmware documentation for recent file renames

firmware_class.c was split into several files under
drivers/base/firmware_loader. The new main.c has the functions which
/request_firmware.rst references.

Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>