staging: wilc1000: refactor SPI read/write commands handling API's

Refactor SPI commands handling by making use of 'struct' for data
exchange and extraction of information flow between host and firmware.
The SPI read/write commands are now handled in separate function instead
of using a single function to process all types of command.
The use of 'struct' helped to make the code self explanatory. These
points were discussed and suggested during code review [1].

1. https://www.spinics.net/lists/linux-wireless/msg191489.htmlSigned-off-by: Ajay Singh <ajay.kathat@microchip.com>
Link: https://lore.kernel.org/r/20200203160848.4052-1-ajay.kathat@microchip.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>