ltq-vdsl-mei: reset g_tx_link_rate on showtime exit

Without this change, ifx_mei_atm_showtime_check() will always return
"showtime" after one call of MEI_InternalXtmSwhowtimeEntrySignal()
was done, even if MEI_InternalXtmSwhowtimeExitSignal() was called
in the meantime.

The ifx_mei_atm_showtime_check() function is used by the ltq-atm and
ltq-ptm driver.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>