net: wireless: bcmdhd: fix RTT report of providing wrong information

adding exception handling for RX rate/bitrate translation in RTT report.
The missing error check resulted in wrong interpretation and it caused
legacy HAL layer crash.

Signed-off-by: Insun Song <insun.song@broadcom.com>
Bug:37505450
Change-Id: I088e20d882b76afd8c8ba5db0390de4df57fd7ea