drivers: i2c: stm32: don't manage cache aside DMA support

Fix STM32 I2C driver to not call cache management related functions
when DMA is not used. This change fixes an issue introduced by
commit 42c3a781 ("i2c: stm32: Add cache memory support"). The issue
makes boards embedding this driver with both CONFIG_I2C_STM32_V2_DMA
and CONFIG_ARCH_MPU disabled to fail to build with an error trace message
like the below:
   .../i2c_ll_stm32_v2.c:701: undefined reference to `mem_attr_check_buf'

Fixes: 42c3a781
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>