Staging: silicom: use kstrtoint_from_user()

The main problem with the hand rolled code was that there weren't any
range checks so you could corrupt memory by writing too much data to
the proc file.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Cotey <puff65537@bansheeslibrary.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>