ipmi: fix return value of ipmi_set_my_LUN

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/char/ipmi/ipmi_msghandler.c: In function 'ipmi_set_my_LUN':
drivers/char/ipmi/ipmi_msghandler.c:1335:13: warning:
 variable 'rv' set but not used [-Wunused-but-set-variable]
  int index, rv = 0;

'rv' should be the correct return value.

Fixes: 048f7c3e ("ipmi: Properly release srcu locks on error conditions")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>