trusty-ipc: fix return value on error path If the transfer type is invalid, the call would incorrectly return 0 rather than an error code. Bug: 400326085 Test: Treehugger Change-Id: I7ae1a31c2ad09e7e682de133e0c144c43f96b7c8 Signed-off-by: Mike McTernan <[email protected]>