i2c: mv64xxx: Fix some build warnings

Some functions and variables are only used if the configuration selects
HAVE_CLK. Protect them with a corresponding #ifdef CONFIG_HAVE_CLK block
to avoid compiler warnings.

Signed-off-by: Thierry Reding <treding@nvidia.com>
[wsa: added marker to #endif]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>