regmap: Fix debugfs-file 'registers' mode

The macro "REGMAP_ALLOW_WRITE_DEBUGFS" can be used to enable write
support on the registers file in the debugfs. The mode of the file is
fixed to 0400 so it is not possible to write the file ever.

This patch fixes the mode by setting it to the correct value depending
on the macro.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>