nvmem: core: Set the provider read-only when no write callback is given

If no write callback is given the device should be marked as read-only.
While at it also move from a bit or to a logical or as that is a logical
expression.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>