NcpSpi: Improve the error handling on spinel SPI tx path (#1484)

In the unlikely case where `otPlatSpiSlavePrepareTransaction()` returns
a failure code it is ensured that the spinel frame is not removed from
the tx buffer and instead the task which prepares the next SPI send
frame is posted again.

src/ncp/ncp_spi.cpp

@@ -289,6 +289,8 @@ ThreadError NcpSpi::PrepareNextSpiSendFrame(void)
     if (errorCode != kThreadError_None)
     {
         mTxState = kTxStateIdle;
+        mPrepareTxFrameTask.Post();
+        ExitNow();
     }
 
     // Remove the frame from tx buffer and inform the base

src/ncp/ncp_spi.hpp

@@ -114,7 +114,6 @@ private:
 
     uint8_t mEmptySendFrame[kSpiHeaderLength];
     uint8_t mEmptyReceiveFrame[kSpiHeaderLength];
-
 };
 
 }  // namespace Thread