1. Remove the extra mask (TIM_DIER(timer_peripheral) & flag)
in timer_get_flag. Not all flags in SR relate to IRQs and the flags that
do might be useful even if interrupts not enabled.
2. In a number of functions (e.g. timer_set_output_idle_state) that only
apply to advanced timers, add test for TIM1 and TIM8, as done in some others.
3. Added timer_force_event to manipulate the EG register
TBD DMA burst - can't get my head around why this would be useful.
Someone must have thought so!
