boot: Switch to flash_area_get_sector.

Replace flash_area_sector_from_off calls with flash_area_get_sector.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/boot_serial/src/boot_serial.c b/boot/boot_serial/src/boot_serial.c
index 1e453d8..e346280 100644
--- a/boot/boot_serial/src/boot_serial.c
+++ b/boot/boot_serial/src/boot_serial.c
@@ -318,7 +318,7 @@
         return start;
     }
 
-    if (flash_area_sector_from_off(end, &sect)) {
+    if (flash_area_get_sector(fap, end, &sect)) {
         return -EINVAL;
     }
 
@@ -437,7 +437,7 @@
          * TODO: This is single occurrence issue, it should get detected during tests
          * and fixed otherwise you are deploying broken mcuboot.
          */
-        if (flash_area_sector_from_off(boot_status_off(fap), &status_sector)) {
+        if (flash_area_get_sector(fap, boot_status_off(fap), &status_sector)) {
             rc = MGMT_ERR_EUNKNOWN;
             BOOT_LOG_ERR("Unable to determine flash sector of the image trailer");
             goto out;