drivers: serial: gecko: Fix build error on Series 0

Series 0 does not have the TXIDLE status flag. The closest equivalent is
TXC, but it isn't set until the first transmission completes, and is
therefore not safe to use in PM suspend without also separately keeping
track of whether the driver has ever initiated a transmission. For now,
disable the TXIDLE check on devices that don't support it as a minimal
fix for the observed build error. This is effectively equivalent to
reverting the addition of PM support for these devices only. Since these
devices don't have a low-power system timer implementation anyway, the
lack of PM handling does not hurt anything.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>