realtek: fix ZYXELS_VERS for GS1900-48

realtek: fix ZYXELS_VERS for GS1900-48

The GS1900-48 firmware image is identified by the 'AAHN' ID, while the
GS1900-48HP is identified by 'AAHO' [1]. The latter was used, resulting
in the following error message when upgrading via the stock web UI:

  Device only can support firmware from V1.00(AAHN.0) and later version

Fix image generation by using the correct ID.

[1] https://download.zyxel.com/GS1900-48/firmware/GS1900-48_2.70(AAHN.3)C0_2.pdf

Link: https://forum.openwrt.org/t/146533
Fixes: 47f5a0a3eed5 ("realtek: Add support for ZyXEL GS1900-48 Switch")
Suggested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Signed-off-by: Sander Vanheule <sander@svanheule.net>