Fix time accounting on msp430 Series 5 MCU based platforms (wismote)

See https://github.com/contiki-os/contiki/pull/727 for explanation
Fix time accounting on TI EXP5438 MSP430x5438a experimenters board
Merge pull request #777 from atiselsts/msp430f5xxx_timing_fixes

Fix time accounting on msp430 Series 5 MCU based platforms