regmap: Reset cache status when reinitialsing the cache

When we reinitialise the cache make sure that we reset the cache access
flags, ensuring that the reinitialised cache is in the default state
which is what callers would and do expect given the function name.

This is particularly likely to cause issues in systems where there was no
cache previously as those systems have cache bypass enabled, as for the
wm8994 driver where this was noticed.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>