i2c: at91: use BIT() macro to define register bits

This patch just fixes typo before applying later patches which will use
register bits with index above 16.

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>