regmap: debugfs: Fix misuse of IS_ENABLED

IS_ENABLED should only be used for CONFIG_* symbols.

I have done a small test:
  #define REGMAP_ALLOW_WRITE_DEBUGFS
  IS_ENABLED(REGMAP_ALLOW_WRITE_DEBUGFS) returns 0.

  #define REGMAP_ALLOW_WRITE_DEBUGFS 0
  IS_ENABLED(REGMAP_ALLOW_WRITE_DEBUGFS) returns 0.

  #define REGMAP_ALLOW_WRITE_DEBUGFS 1
  IS_ENABLED(REGMAP_ALLOW_WRITE_DEBUGFS) returns 1.

  #define REGMAP_ALLOW_WRITE_DEBUGFS 2
  IS_ENABLED(REGMAP_ALLOW_WRITE_DEBUGFS) returns 0.

So fix the misuse of IS_ENABLED(REGMAP_ALLOW_WRITE_DEBUGFS) and switch to
use #if defined(REGMAP_ALLOW_WRITE_DEBUGFS) instead.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>