The timer_clear function doesn't clear the hardware timer counter, but
is designed to clear the allocation of the channel. The consequence is
that the IFS timer here is not set to zero in the callback, but only
stopped at the current value. When the timer is started again, it has to
count the full timer range until it matches the timeout value again.
This commit fixes this issue by using timer_set instead of
timer_set_absolute. This way the current timer value (when the timer is
stopped) is read and the IFS timeout value is added to the current timer
value.
ieee802.15.4 specifies 40 symbols as LIFS value and 12 symbols as SIFS
value. Furthermore, the 2.4Ghz DSSS mode has a symbol rate of
62.5Ksymbols/s. To have the LIFS and SIFS in the code match the timings
from the specification, the TIMER_FREQ must match the symbol rate of
62.5Ksymbol/s such that one tick of the timer equals one symbol in time.
