i2c: imx-lpi2c: Don't open-code DIV_ROUND_UP

There is no change in the generated code (tested on an ARCH=arm
allmodconfig build).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>