semihosting: Adjust error checking in common_semi_cb

semihosting: Adjust error checking in common_semi_cb

The err parameter is non-zero if and only if an error occured.
Use this instead of ret == -1 for determining if we need to
update the saved errno.

This fixes the errno setting of SYS_ISTTY, which returns 0 on
error, not -1.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>