cmd_sf: Fix compiler warning

cmd_sf.c: In function 'do_spi_flash':
cmd_sf.c:164:9: warning: 'skipped' may be used uninitialized in this function

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Simon Glass <sjg@chromium.org>