drivers: nsos: fix ENOMEM return from ZFD_IOCTL_POLL_PREPARE

ioctl() API expects error codes to be returned as negative value directly,
instead of using 'errno'. Fix ZFD_IOCTL_POLL_PREPARE handling to respect
that.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>