ACPICA: FADT: Remove extraneous warning for very large GPE registers

This change removes a size mismatch warning if the legacy
length field for a GPE register set is larger than the 64-bit
GAS structure can accomodate. GPE register sets can be larger
than the 255 bit limitation of the GAS structure. Linn Crosetto
(linn@hp.com).

Signed-off-by: Linn Crosetto <linn@hp.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>