Fix bug in gdb.ada/bias.exp

While working on a different bug in the Ada support, I found that the
gdb.ada/bias.exp test is slightly incorrect.  In particular, it is
using a range type, which it then overflows during an operation.

This patch changes the test so that the computed values remain in
range.

gdb/testsuite/ChangeLog
2020-11-10  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/bias.exp: Update.
	* gdb.ada/bias/bias.adb (X): Change value.