drivers: i2c_ll_stm32_v2: Use the correct flags in event ISR

Event ISR checks if the TX/RX interrupts is enabled instead
of the TXIS/RXNE interrupt status flags. Use the TXIS/RXNE
interrupt status flags to check which interrupt event
happened.

Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>
Signed-off-by: Daniel Wagenknecht <wagenknecht@clage.de>