staging: wilc1000: Use crc7 in lib/ rather than a private copy

The code in lib/ is the desired polynomial, and even includes
the 1-bit left shift in the table rather than needing to code
it explicitly.

While I'm in Kconfig, add a description of what a WILC1000 is.
Kconfig questions that require me to look up a data sheet to
find out that I probably don't have one are a pet peeve.

Cc: Adham Abozaeid <adham.abozaeid@microchip.com>
Cc: linux-wireless@vger.kernel.org
Reviewed-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: George Spelvin <lkml@sdf.org>
Link: https://lore.kernel.org/r/20200326152251.19094-1-ajay.kathat@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>