s390/netiucv: improve checking of sysfs attribute buffer

High values are always wrong for netiucv's sysfs attribute "buffer".
But the current code does not detect values between 2**31 and 2**32
as invalid. Choosing type "unsigned int" for variable "bs1" and making
use of "kstrtouint()" improves the syntax checking for "buffer".

Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>