drivers: i2c_ll_stm32_v1: Disable/enable only Buffer interrupts

The EVENT interrupt is generated when:
– SB = 1
– ADDR = 1
– ADD10= 1
– BTF = 1 with no TxE or RxNE event
– TxE event to 1 if Buffer interrupt enabled
– RxNE event to 1 if Buffer interrupt enabled

We need to disable and enable the Buffer TxE and RxNE interrupt
inside the EVENT ISR. The LL_I2C_{Enable,Disable}IT_TX and
LL_I2C_{Enable,Disable}IT_RX functions, used now, enable and
disable both Buffer and Event interrupt. This patch uses the
LL_I2C_EnableIT_BUF and  LL_I2C_DisableIT_BUF functions to
enable and disable the Buffer interrupt (TxE, RxNE).

Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>