i2c-at91: compile fix (IS_ERR)

  CC      drivers/i2c/busses/i2c-at91.o
drivers/i2c/busses/i2c-at91.c: In function 'at91_i2c_probe':
drivers/i2c/busses/i2c-at91.c:213: warning: implicit declaration of function 'IS_ERR'

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: David Brownell <david-b@pacbell.net>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>