[FX3] When in FPGA load and RF mode, transitions to U1/U2 should be rejected

This pull request fixes #80.  The core problem can be understood by reading the CyU3PUsbLPMReqCb_t documentation in the FX3APIGuide.  Basically some USB3.0 controllers after completing "all work" will request a low power mode.  In the case of the bladeRF when in either FPGA load or RF mode, suspending is counter productive.  So this patch causes the FX3 to reject U1/U2 requests when in FPGA or RF mode.

This patch does not cause USB 3.0 CV to complain, so I think this is a compliant change.