gpio: Correct handling of 'gpio status'

This is broken - we need to look at the first two characters to distinguish
'gpio status' from 'gpio set'.

Fixes: 0ffe6ab5 (gpio: Allow 's' as an abbreviation for 'status')
Reported-by: Soeren Moch <smoch@web.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Soeren Moch <smoch@web.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Hannes Schmelzer <oe5hpm@oevsv.at>