default rxtx binary mode configuration reflects bitmode value

[Algafix]

Doing:

bladeRF> set frequency rx 1575.42M
bladeRF> set bandwidth rx 20M
bladeRF> set samplerate rx 20M
bladeRF> set bitmode 8
bladeRF> rx config file=test_8bits.bin n=1200M timeout=60000

Does not set the file format to 8 bits:

bladeRF> rx

  State: Idle
  Channels: RX1
  Last error: None
  File: test_8bits.bin
  File format: SC16 Q11, Binary
  # Samples: 1258291200
  # Buffers: 32
  # Samples per buffer: 32768
  # Transfers: 16
  Timeout (ms): 60000

That's because the file format is updated inside the rxtx_str2fmt function only if the format option is provided on the rx/tx config command. I think it makes sense to use as default SC8Q7 if the user specifies 8 bits instead of the default SC16Q11.

My idea was to update the default mode when the set bitmode command is processed, but when looking at how the code is organized I thought this was a better idea. Let me know if you are ok with it or would prefer another approach.

Thanks!