trusty:storageproxyd: Fix return paths on errors

The function send_ufs_rpmb_req is missing return paths on errors.
This patch fixes it so that any UFS command failure will return
error code to the function caller.

Bug: 193855098
Test: Trusty storage tests
Merged-In: I391ecff9ed3f892b7c3adae0ceeb18930791326f
Change-Id: I391ecff9ed3f892b7c3adae0ceeb18930791326f