sm8450-common: Unmount /firmware after fastbootd starts

Fixes `fastboot flash modem modem.img`.

Change-Id: I17e2183f666979c94c3f5b1affd660bfd60df15d

rootdir/etc/init.recovery.qcom.rc

@@ -48,3 +48,6 @@ on fs
 
 on property:dev.mnt.blk.firmware=*
     write /sys/kernel/boot_adsp/boot 1
+
+on property:init.svc.fastbootd=running
+    umount /firmware