ipmi_si: Use proper ACPI macros to check error code for failures

Instead of direct comparison, use proper ACPI macros to check error code
for failures.

While at it, drop unneeded 'else' keyword.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Message-Id: <20210402174334.13466-5-andriy.shevchenko@linux.intel.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>