Fix the decode_get_sensor_reading_resp method

In the decode_get_sensor_reading_resp method, sensor_data_size is a
parameter of type out, so before assignment, it cannot be compared
with the sensor_data_size parameter in the response.

Resolves : https://github.com/openbmc/pldm/issues/28



Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: If6c9ecb5ef954c6e7da48d2fda77adddc5f2202b