i2c: at91: fix code checker warnings

buf_len is a size_t, so unsigned, but was tested with '<= 0'.

Reported-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>