regmap: Silence warning on invalid zero length read

Zero length reads make no sense in a regmap context and are likely to
trigger bugs further down the stack so insert an error check, also
silencing compiler warnings about use of ret in cases where we iterate
per register.

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>