Unfortunately the device_path char* is dynamically allocated
and #defined and const char *'ed depending on how each platform provided the
uart device paths which makes cleaning it up more complicated.
Added a check for the IEI_TANK.
Removed the char** (in favor of a char*) for mraa_find_uart_bus_pci.)
Signed-off-by: Noel Eck <noel.eck@intel.com>