regmap: Allow missing device in regmap_name_read_file()

This fixes a possible NULL pointer dereference oops in
regmap_name_read_file() when the regmap does not have a device
associated with it. For example syscon regmaps retrieved with
syscon_regmap_lookup_by_compatible() don't have a device.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Mark Brown <broonie@kernel.org>