test/py: Have test_usb.py raise an Exception with unsupported filesystems

With a newer pylint we get a warning about how offset could be used
before assigned. This is because when the underlying filesystem wasn't
one that is supported we would have runtime test failures. Address this
by raise'ing an Exception if fs is not supported.

Signed-off-by: Tom Rini <trini@konsulko.com>

test/py/tests/test_usb.py

@@ -580,6 +580,8 @@ def test_usb_load(u_boot_console):
                     elif fs in ['ext4', 'ext2']:
                         file, size, expected_crc32 = \
                             usb_ext4load_ext4write(u_boot_console, fs, x, part)
+                    else:
+                        raise Exception('Unsupported filesystem type %s' % fs)
 
                     offset = random.randrange(128, 1024, 128)
                     output = u_boot_console.run_command(