greybus: gb-beagleplay: Fix timeout handling in bootloader functions

greybus: gb-beagleplay: Fix timeout handling in bootloader functions

wait_for_completion_timeout() returns the remaining jiffies
(at least 1) on success or 0 on timeout, but never negative
error codes. The current code incorrectly checks for negative
values, causing timeouts to be ignored and treated as success.

Check for a zero return value to correctly identify and
handle timeout events.

Fixes: 0cf7befa3ea2 ("greybus: gb-beagleplay: Add firmware upload API")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Link: https://patch.msgid.link/20251121064027.571-1-vulab@iscas.ac.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>