[bsp][stm32] fix LTDC_IRQHandler irq enter bug.
[bsp][stm32] fix LTDC_IRQHandler irq enter bug.

geniusgogo/rt-thread,hezlog/rt-thread,hezlog/rt-thread,nongxiaoming/rt-thread,geniusgogo/rt-thread,hezlog/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,ArdaFu/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,geniusgogo/rt-thread,geniusgogo/rt-thread,hezlog/rt-thread,geniusgogo/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,geniusgogo/rt-thread,RT-Thread/rt-thread,ArdaFu/rt-thread,RT-Thread/rt-thread,ArdaFu/rt-thread,hezlog/rt-thread,hezlog/rt-thread,nongxiaoming/rt-thread,hezlog/rt-thread,geniusgogo/rt-thread,ArdaFu/rt-thread,ArdaFu/rt-thread,nongxiaoming/rt-thread,ArdaFu/rt-thread,RT-Thread/rt-thread,ArdaFu/rt-thread