[aes/rtl] Select clear/all-zero for invalid/default selector inputs
[aes/rtl] Select clear/all-zero for invalid/default selector inputs

In case of invalid/default selector inputs, muxes now select the
clear/all-zero signal if available rather than just the first listed
signal.

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan