rp2/modules: Fix FatFS boot script to detect invalid FAT filesystem.

This change helps detect if the filesystem is invalid, by also including
the first mount attempt within the try-except.  Then the FAT is reformatted
if needed.

Fixes issue #15779.

Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

ports/rp2/modules/_boot_fat.py

@@ -5,10 +5,9 @@ import machine, rp2
 # Try to mount the filesystem, and format the flash if it doesn't exist.
 bdev = rp2.Flash()
 try:
-    fs = vfs.VfsFat(bdev)
+    vfs.mount(vfs.VfsFat(bdev), "/")
 except:
     vfs.VfsFat.mkfs(bdev)
-    fs = vfs.VfsFat(bdev)
+    vfs.mount(vfs.VfsFat(bdev), "/")
-vfs.mount(fs, "/")
 
-del vfs, bdev, fs
+del vfs, bdev