libbladeRF: Submit BLADERF_STREAM_SHUTDOWN without waiting on stream

Normally, we want to wait for the stream to be running before
submitting a buffer. However, BLADERF_STREAM_SHUTDOWN is a special
case; we want to always submit this immediately so that we can
end a stream, even if it's not currently running.

This commit fixes #244.
libbladeRF: Support NULL 'actual' argument for bladerf_set_bandwidth()

This function now correctly handles the case where a caller specifies
NULL for the 'actual' parameter if (s)he does not wish to retrieve
the actual bandwidth set.

This fixes #245.
WIP: Modify the RXVGA2 calibration routines.
Calibration update.
libbladeRF: Moved LPF DC tuning value write into address loop

This is just for readability. There's no functional change, since
there's only 1 address to loop over anyway.
bladeRF-cli: Workaround - Added retry loop to TXLPF calibration

The TXLPF calibration appears to fail on the first two attempts to
run it. As a temporary workaround, a retry loop has been added
around attempts to run this calibration.
libbladeRF: Stubbed out autocal test program
Use linear interp to get first guess at DC offset values
Integer interpolation and apply rx auto cal parameters
Touched up output