realtek: fix zyxel-vers usage for XGS1250-12

realtek: fix zyxel-vers usage for XGS1250-12

Commit daefc646e6d4 ("realtek: fix ZyXEL initramfs image generation")
fixed a shell expansion issue with zyxel-vers usage. Commit 045baca10b15
("realtek: deduplicate GS1900 recipes") took care of this for the
rtl838x and rtl839x subtargets, but the single device officially
supported in rtl930x - the XGS1250-12 - was overlooked. This commit
updates the XGS1250-12 build recipe as well.

Signed-off-by: Stijn Segers <foss@volatilesystems.org>
(cherry picked from commit 557db5106ce7809a5b2f1ec6d2407c1f32d10ef7)