regmap: fix the kerneldoc for regmap_test_bits()

The kerneldoc comment for regmap_test_bits() says that it returns -1 on
regmap_read() failure. This is not true - it will propagate the error
code returned by regmap_read(). Fix it.

Fixes: aa2ff9db

 ("regmap: provide helpers for simple bit operations")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200607093421.22209-1-brgl@bgdev.pl
Signed-off-by: Mark Brown <broonie@kernel.org>