i2c-davinci: Fix lost interrupt

DAVINCI_I2C_STR_REG is a write 1 to clear register,
so don't use a read/modify/write cycle.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Dirk Behme <dirk.behme@gmail.com>
Signed-off-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>