[BACKPORT] stm32f40xxx_i2c isr processing

   Due to some errant condition (I2C error or code)
   The ISR was being rentered until the end of the
   frame. Causing other ISR to not run for 280 uS.

   Backport of Alexander Oryshchenko comits
   d1cfcab, 643234d, 9b812e3 fixes this condition.