stm32l4/flashpage: fix page erase

- The PNB in FLASH_CR wasn't cleared before every erase operation
  and the new value was just stacked on top. After a couple of erase
  the PNB written was overlapping with old ones failing to erase the
  correct page.

cpu/stm32_common/periph/flashpage.c

@@ -111,6 +111,7 @@ static void _erase_page(void *page_addr)
     }
     pn = (uint8_t)page;
 #endif
+    CNTRL_REG &= ~FLASH_CR_PNB;
     CNTRL_REG |= (uint32_t)(pn << FLASH_CR_PNB_Pos);
     CNTRL_REG |= FLASH_CR_STRT;
 #else /* CPU_FAM_STM32F0 || CPU_FAM_STM32F1 */