spl: semihosting: Don't close fd before spl_load_simple_fit

spl: semihosting: Don't close fd before spl_load_simple_fit

On real hardware, semihosting calls tend to have a large constant
overhead (on the order of tens of milliseconds). Reduce the number of
calls by one by reusing the existing fd in smh_fit_read, and closing it
at the end of spl_smh_load_image.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>