Staging: silicom: add some range checks to proc functions

If you tried to cat more than 255 characters (the last character is for
the terminator) to these proc files then it would corrupt kernel memory.

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>