i2c: at91: fix code checker warnings
i2c: at91: fix code checker warnings

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

Reported-by: Wolfram Sang <fd4ce474653598159cad06f3c83387a05cd53a44@the-dreams.de>
Signed-off-by: Cyrille Pitchen <e50fad4ce891c9ba2f33288a3659adde668ff62d@atmel.com>
Acked-by: Ludovic Desroches <627575320a9526cb6c91021c9940202b0e84c416@atmel.com>
Signed-off-by: Wolfram Sang <fd4ce474653598159cad06f3c83387a05cd53a44@the-dreams.de>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs