drivers: stm32_fmc2_nand: fix incorrect error detection

Clear interrupt flag register after each sector read to avoid
issue when checking the register status.
Without clearing the interrupt, the status read doesn't wait
properly the ready bit.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: If290e3f165b986f0e736bb1b5e4d3dad4b749d74