[nrf51822] avoid using a global variable and fix I2C read sequence

+ replace global variable - `addrSet`  with struct variable - `address_set`
+ I2C read sequence enables suspend task to prevent a received byte to be overridden
+ properly generate stop condition of I2C read sequence. Originally, stop condition may delay a few clocks, extra bytes will be read. When using sensors like MPU6050, it is an issue.