[HAL][KSDK] Fix first serial char not being sent

The K64F/K22F didn't send the first char. I don't know why exactly, but
any type of reading a status register seems to fix it. And this extra
line at least makes sense. Now it works correctly.
Merge pull request #677 from Sissors/ksdk

Targets: Freescale KSDK - Fix first serial char not being sent