cpu/kinetis_common: periph/timer, read PIT value before stopping

Reading CVAL after stopping timer seem to return zero.
Ref. manual states: "The counter period can be restarted, by first
disabling, and then enabling the timer with TCTRLn[TEN]", but does not
state whether the reset of CVAL happens on TEN 0->1 transition, or 1->0
transtion. Empirical evidence suggests the latter is the implemented behaviour.