cpu/stm32f3: set i2c clk input to sysclk when HSI is deactivated

According to the STM32F3 clocktree (ref. manual p. 126) the I2CX
hardware is driven by either HSI or SYSCLK.
If HSI is deactivated SYSCLK clock must be selected as I2CX clock.

Signed-off-by: Steffen Pengel <steffen.pengel@gmail.com>