net: lib: lwm2m: Use int16_t for signal quality

RSRQ is the ratio between send and received signal strength and usually
understood/expected to be represented as a ratio in dB and as such always
has a negative range. So to allow RSRQ to be represented correctly the
resource must allow negative values, but currently it's limited to unsigned
8bit integers.

Signed-off-by: Henning Fleddermann <henning.fleddermann@grandcentrix.net>