i2c, avr32: fix compiler warning "input is not relaxable"

i2c, avr32: fix compiler warning "input is not relaxable"

compiling U-Boot for avr32 boards shows since
commit 3d1957f0ea01 "dm: i2c: Add support for multiplexed I2C buses"
this warning:

Building current source for 4 boards (4 threads, 8 jobs per thread)
     avr32:  +   atstk1002
+(atstk1002) drivers/i2c/built-in.o: warning: input is not relaxable
     avr32:  +   grasshopper
+(grasshopper) drivers/i2c/built-in.o: warning: input is not relaxable
     avr32:  +   atngw100
+(atngw100) drivers/i2c/built-in.o: warning: input is not relaxable
     avr32:  +   atngw100mkii
+(atngw100mkii) drivers/i2c/built-in.o: warning: input is not relaxable
    0    4    0 /4      0:00:16  : atngw100mkii

Fix it.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Roger Meier <r.meier@siemens.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>