ipmi: initialize len variable

Clang static analysis reports this issue
ipmi_ssif.c:1731:3: warning: 4th function call
  argument is an uninitialized value
  dev_info(&ssif_info->client->dev,
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The 4th parameter is the 'len' variable.
len is only set by a successful call to do_cmd().
Initialize to len 0.

Signed-off-by: Tom Rix <trix@redhat.com>
Message-Id: <20220320135954.2258545-1-trix@redhat.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>