[gdb/testsuite] Fix gdb.mi/gdb2549.exp with check-read1

When running test-case gdb.mi/gdb2549.exp with check-read1, I run into:
...
FAIL: gdb.mi/gdb2549.exp: register values x (timeout)
...

Fix this by applying the same fix as for "register values t" in commit
478e490a4df "[gdb/testsuite] Fix gdb.mi/gdb2549.exp with check-read1".

Tested on x86_64-linux.