tiny-printf: Correct the snprintf() implementation

This current code passes the variable arguments list to sprintf(). This is
not correct. Fix it by calling _vprintf() directly.

This makes firefly-rk3288 boot again.

Fixes: abeb272d

 ("tiny-printf: Support snprintf()")
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>