logging: Increase package size limit to 2047

Package length was previously stored on 10 bits which limited
package size to 1023 bytes. Descriptor which contained package
length field had one unused bit which can be used to increase
package length field to 11 bits doubling the maximum package
length.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>