regmap: debugfs: Factor out debugfs_tot_len calc into a function

In preparation to support the regmap debugfs ranges functionality
factor this code out to a separate function.  We'll need to ensure
that the value has been correctly calculated from two separate places
in the code.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>