i2c: tegra: pass proper name for getting clock

Tegra's i2c controller require two clock sources named as
div_clk and fast_clk.
This change make sure that driver pass the correct clock's
name when it acquires clock handle. Also change the
variable name to reflect the correct clock handles.

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