IsaBridgeEC: keep unrelated decode bits when enabling the AMD bridge

For the enable states amd_config_make_mask set IoMemPortDecodeEnable to
a bare 0x20. mask64on32 takes the high half of the mask as the bits to
keep from the old value, so a mask of 0x20 means (old & 0) | 0x20, which
wrote D14F3x48 as exactly 0x20 and cleared every other decode enable in
it, including the unrelated SIO, RTC, wide IO and legacy ones.

Set the keep half to ~0x20 so only the decode enable bit is forced and
everything else is preserved. This also repairs amd_config_is_enabled,
which shares the mask and previously only reported enabled when the whole
register happened to equal 0x20.