stm32f7:Serial fix for dropped data

  1) Revert the inherited dma bug from the stm32
     see https://bitbucket.org/nuttx/nuttx/commits/df9ae3c13fc2fff2c21ebdb098c520b11f43280d
     for details.

  2) Most all CR1-CR3 settings can not be configured while UE
     is true. Threfore we make all operation atomic and disable
     UE and restore it's originalstate on exit.