x86/acpi: Correct out-of-date comment of __acpi_map_table()

The implementation of function __acpi_map_table() has been
changed long time ago, and now it directly invokes
early_ioremap() to setup the temporarily acpi table mappings.

So correct its out-of-date comment.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: len.brown@intel.com
Cc: pavel@ucw.cz
Cc: rjw@sisk.pl
Link: http://lkml.kernel.org/r/51EE7F1C.9020506@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>