vc/google/chromeos: use fw_config field to return sar filename

Use fw_config field to return sar filename instead of fw_config probe.
Return filename unconditionly because the sar_id must be valid in
CBI. If invalid sar_id, the file won't exist in CBFS by design.

Signed-off-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Change-Id: I7b75c5d4fd3c459ad7232bb16c6218a6218f1f77
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70896
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Reviewed-by: Ivy Jian <ivy.jian@quanta.corp-partner.google.com>

src/include/sar.h

@@ -2,6 +2,7 @@
 #ifndef _SAR_H_
 #define _SAR_H_
 
+#include <fw_config.h>
 #include <stdint.h>
 
 #define MAX_ANT_GAINS_REVISION	2
@@ -88,4 +89,6 @@ int get_wifi_sar_limits(union wifi_sar_limits *sar_limits);
 
 const char *get_wifi_sar_cbfs_filename(void);
 
+char *get_wifi_sar_fw_config_filename(const struct fw_config_field *field);
+
 #endif /* _SAR_H_ */