ACPICA: Fix buffer allocation issue for generic_serial_bus region accesses.

The size of the buffer allocated for generic_serial_bus region access
is not correct.  This patch introduces acpi_ex_get_serial_access_length()
to be invoked to obtain correct data buffer length.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Reported by: Lan Tianyu <tianyu.lan@intel.com>
Acked-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>