libbladeRF: Return to IF_NULL when closing a device

This has been found to be necessary on OSX. Without changing back to the
NULL interface, we run into upon re-opening the device.
libbladeRF: Switch back to IF_CONFIG or IF_RF_LINK after flash ops

We need to switch out of IF_SPI_FLASH after flash operations in order to
continue using the FPGA.

This, in conjunction with firmware v1.6.1, fixes #196.