sar: Fix semantics of `get_wifi_sar_cbfs_filename()`

sar: Fix semantics of `get_wifi_sar_cbfs_filename()`

Currently, if `get_wifi_sar_cbfs_filename()` returns NULL, then
`get_wifi_sar_limits()` assumes that the default filename is used for
CBFS SAR file. This prevents a board from supporting different models
using the same firmware -- some which require SAR support and some
which don't.

This change updates the logic in `get_wifi_sar_limits()` to return
early if filename is not provided by the mainboard. In order to
maintain the same logic as before, current mainboards are updated to
return WIFI_SAR_CBFS_DEFAULT_FILENAME instead of NULL in default
case.

Change-Id: I68b5bdd213767a3cd81fe41ace66540acd68e26a
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51485
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>