cpu/stm32f1: fix i2c master read for N>2 bytes

This fixes reading more than 2 bytes from the slave device.
In the current implementation the last byte was not read from
data register and the termination sequence buggy.
cpu/stm32f1: fix i2c read for N=2 bytes
Merge pull request #1872 from thomaseichinger/stm32f1-i2c-multibyte-fix

cpu/stm32f1: fix i2c master read for N=2 and N>2 bytes