zephyr: single_loader: Switch to flash_area_get_sector

Use flash_area_get_sector instead of flash_area_sector_from_off.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/zephyr/single_loader.c b/boot/zephyr/single_loader.c
index 253015e..97c212a 100644
--- a/boot/zephyr/single_loader.c
+++ b/boot/zephyr/single_loader.c
@@ -350,7 +350,7 @@
     memset(&_bs, 0, sizeof(struct boot_status));
 
     /* Get size from last sector to know page/sector erase size */
-    rc = flash_area_sector_from_off(boot_status_off(fa_p), &sector);
+    rc = flash_area_get_sector(fap, boot_status_off(fa_p), &sector);
 
 
     image_index = BOOT_CURR_IMG(state);