i2c: imx-lpi2c: fix type char overflow issue when calculating the clock cycle

Claim clkhi and clklo as integer type to avoid possible calculation
errors caused by data overflow.

Fixes: a55fa9d0

 ("i2c: imx-lpi2c: add low power i2c bus driver")
Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Signed-off-by: Carlos Song <carlos.song@nxp.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>