regmap: debugfs: Simplify calculation of `c->max_reg'

We don't need to use any of the file position information
to calculate the base and max register of each block.  Just
use the counter directly.

Set `i = base' at the top to avoid GCC flow analysis bugs.  The
value of `i' can never be undefined or 0 in the if (c) { ... }.

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