firmware: rename struct fw_priv->fw_id to fw_name

This makes it clear exactly what the field is for. With fw_id it
was not clear to a reader if this was some sort of private concoction
of some sort.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>