GBA link cable:
&nbsp---^---
| 1 3 5 |
| 2 4 6 |
&nbsp-------

SNES controller:
( 1 2 3 4 / 5 6 7 )

Link cable to controller:
1 -> 1 (Vcc)
2 -> 3 (SO -> Latch)
3 -> 4 (SI -> Data)
5 -> 2 (SC -> Clk)
6 -> 7 (GND)

Bin file: SNESAdvance.bin
Modified sources (from 0.1F): io.s main.c
Example code: snespad.c
Button test: snestest.gba

!! Disclaimer !!
Results may vary depending on the controller model used. SNES controllers normally 
operate at 5V, while the GBA's link port only puts out 3.3V, so it's possible that
some controllers may not work.