spi: spidev_test: add option to continuously transfer data - v2

[Frode Isaksen <fisaksen@xxxxxxxxxxxx>]

v2 changes:
- fix bug, hex_dump() dumped tx instead of rx buffer.
- indentation fixes in for() loop and limit long lines.
- exit(1) instead of exit(-1) in case of error.
- simplify function show_transfer_rate().

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