platform/mellanox: mlxreg-io: Fix argument base in kstrtou32() call

Change kstrtou32() argument 'base' to be zero instead of 'len'.
It works by chance for setting one bit value, but it is not supposed to
work in case value passed to mlxreg_io_attr_store() is greater than 1.

It works for example, for:
echo 1 > /sys/devices/platform/mlxplat/mlxreg-io/hwmon/.../jtag_enable
But it will fail for:
echo n > /sys/devices/platform/mlxplat/mlxreg-io/hwmon/.../jtag_enable,
where n > 1.

The flow for input buffer conversion is as below:
_kstrtoull(const char *s, unsigned int base, unsigned long long *res)
calls:
rv = _parse_integer(s, base, &_res);

For the second case, where n > 1:
- _parse_integer() converts 's' to 'val'.
  For n=2, 'len' is set to 2 (string buffer is 0x32 0x0a), for n=3
  'len' is set to 3 (string buffer 0x33 0x0a), etcetera.
- 'base' is equal or greater then '2' (length of input buffer).

As a result, _parse_integer() exits with result zero (rv):
	rv = 0;
	while (1) {
		...
		if (val >= base)-> (2 >= 2)
			break;
		...
		rv++;
		...
	}

And _kstrtoull() in their turn will fail:
	if (rv == 0)
		return -EINVAL;

Fixes: 5ec4a8ac

 ("platform/mellanox: Introduce support for Mellanox register access driver")
Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Link: https://lore.kernel.org/r/20210927142214.2613929-2-vadimp@nvidia.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>