gc_bba_test: fix inverted EXI error checks; builds clean

libogc's EXI functions return '1 on success, <=0 on error', but every call
site tested '== 0'.  A -1 error return therefore read as success, so a failed
EXI_Lock or EXI_Select would fall through and the subsequent transfer would
produce garbage reported as a misleading test failure rather than an EXI
error.  All six call sites now test <= 0.

Also retries EXI_Lock for ~100 ms: it fails outright rather than blocking when
another subsystem holds the channel, and Swiss or a libogc driver may hold it
briefly -- without the retry a transient collision surfaces as a test failure.

Builds clean with devkitPPC 16.1.0 / GCC 16, no warnings: 172 KB .dol.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

gc_bba_test/README.md

@@ -65,3 +65,10 @@ fault. `bba_read()` clocks exactly `len`; don't "round up" for convenience.
 **TXDATA takes one byte per transaction.** The register file's effective
 address auto-increments per data byte, so a 4-byte write to 0x48 would spill
 into 0x49–0x4B. `bba_tx_bytes()` does it correctly.
+
+## Build status
+
+Builds clean (no warnings) with devkitPPC 16.1.0 / GCC 16 in the devcontainer:
+`gc_bba_test.dol`, ~172 KB, entry `0x80003100`.
+
+`.gitignore` excludes the build products — run `make` to produce the `.dol`.