fix automatic filter bandwidth selection

[mossmann]

There was a bug in setBandwidth() in which a value of zero would be turned into a nonzero value by hackrf_compute_baseband_filter_bw(). This made (_current_bandwidth > 0) always true, resulting in the baseband filter bandwidth being set to the minimum (nominally 1.75 MHz) instead of being automatic. Although setSampleRate() did set automatic filter bandwidth correctly, it was subsequently set incorrectly by setBandwidth().

I removed the call to hackrf_compute_baseband_filter_bw() as hackrf_set_baseband_filter_bandwidth() does this for you.

I also removed the automatic bandwidth filter setting in setSampleRate() as hackrf_set_sample_rate() does this for you.

[mossmann]

This may fix #23, although I can't be certain without more information from the user.