wifi_fw_mount: check mount return value instead of relying on mdt

Now wifi_fw_mount script expects q6_fw.mdt file
if partition mounted. But IPQ5332 soc doesn't have
q6_fw.mdt file, it has q6_fw0.mdt & q6_fw1.mdt.
so these mdt files vary based on soc.

Instead of relying on mdt files, this patch will
check mount return value.

Change-Id: Ic7c31333b9b3c3afe082ae699fe5a191bf575b4d
Signed-off-by: Manikanta Mylavarapu <quic_mmanikan@quicinc.com>