i2c: pnx: fix runtime warnings caused by enabling unprepared clock

The driver can not be used on a platform with common clock framework
until clk_prepare/clk_unprepare calls are added, otherwise clk_enable
calls will fail and a WARN is generated.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>