OGL: Don't leave staging texture buffer bound after mapping

This could cause glReadPixels() calls which assume no buffer is bound
(e.g. CPU EFB access) to fail. The problem was limited to devices which
don't support persistent mapping, as the map path is not otherwise.

Source/Core/VideoBackends/OGL/OGLTexture.cpp

@@ -511,10 +511,8 @@ bool OGLStagingTexture::Map()
     flags = GL_MAP_READ_BIT | GL_MAP_WRITE_BIT;
   glBindBuffer(m_target, m_buffer_name);
   m_map_pointer = reinterpret_cast<char*>(glMapBufferRange(m_target, 0, m_buffer_size, flags));
-  if (!m_map_pointer)
-    return false;
-
-  return true;
+  glBindBuffer(m_target, 0);
+  return m_map_pointer != nullptr;
 }
 
 void OGLStagingTexture::Unmap()