regmap: fix writes to non incrementing registers

commit 2e31aab0 upstream.

When checking if a register block is writable we must ensure that the
block does not start with or contain a non incrementing register.

Fixes: 8b9f9d4d ("regmap: verify if register is writeable before writing operations")
Signed-off-by: Ben Whitten <ben.whitten@gmail.com>
Link: https://lore.kernel.org/r/20200118205625.14532-1-ben.whitten@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>