Fix for 1542C support, it turns out that the 0x8 extended bios flag is only
set when extended translation is turned on, thus we need to do the mailbox
unlock command no matter what value is in the extended bios flag byte as
the other extensions (ie, > 2 drive support) cause the same problems.

The code has been changed to ALWAYS unlock the mailbox interface on ALL
1542C class boards.