i2c: tegra: Remove NULL pointer check before clk_enable/disable/prepare/unprepare

clk_enable, clk_disable, clk_prepare, and clk_unprepare APIs have
implementation for checking clk pointer not NULL and clock consumers
can safely call these APIs without NULL pointer check.

So, this patch cleans up Tegra i2c driver to remove explicit checks
before these APIs.

Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>