i2c: tegra: use clk_disable_unprepare in place of clk_disable

Use clk_disable_unprepare() inplace of clk_disable().
This was missed as part of moving clock enable/disable to
prepare/unprepare for using the common clock framework.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>