qlogic: check kstrtoul() for errors

[ Upstream commit 5fc853cc ]

We accidentally left out the error handling for kstrtoul().

Fixes: a520030e ("qlcnic: Implement flash sysfs callback for 83xx adapter")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>