serial: 8250_exar: Adjust IOT2000 matching

Since there are more IOT2040 variants with identical hardware but
different asset tags, the asset tag matching should be adjusted to
support them.

As only the IOT2040 variants have the Exar chip on board, matching on
their board name is enough. In the future there will be no other devices
with the "SIMATIC IOT2000" DMI board name but different hardware.

Signed-off-by: Su Bao Cheng <baocheng.su@siemens.com>
Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>