arch: stm32: fix undeclared page variable in flash write

Fix a compilation error in stm32f10xxf30xx_flash.c where the variable
`page` is used without being declared in up_progmem_write().

The issue appears when STM32_FLASH_DUAL_BANK is enabled.

The page index is now derived from the write address using
up_progmem_getpage() and used for bank selection.

Signed-off-by: Adwait Godbole <adwaitngodbole@gmail.com>

Author: adwait-godbole

arch/arm/src/stm32/stm32f10xxf30xx_flash.c

@@ -291,8 +291,11 @@ ssize_t up_progmem_write(size_t addr, const void *buf, size_t count)
   uintptr_t base;
   uint16_t *hword = (uint16_t *)buf;
   size_t written = count;
+  size_t page;
   int ret;
 
+  page = up_progmem_getpage(addr);
+
 #if defined(STM32_FLASH_DUAL_BANK)
   /* Handle paged FLASH */