ipmi: kcs_bmc: handle devm_kasprintf() failure case

devm_kasprintf() may return NULL if internal allocation failed so this
assignment is not safe. Moved the error exit path and added the !NULL
which then allows the devres manager to take care of cleanup.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Fixes: cd2315d4 ("ipmi: kcs_bmc: don't change device name")
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Haiyue Wang <haiyue.wang@linux.intel.com>