regmap: Drop check whether a register is readable in regcache_read

One of the reasons for using a cache is to have a software shadow of a register
which is writable but not readable. This allows us to do a read-modify-write
operation on such a register.

Currently regcache checks whether a register is readable when performing a
cached read and returns an error if it is not. Drop this check, since it will
prevent us from using the cache for registers where read-back is not possible.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>