ath10k: fix the garage chars in board file name creation

The variant[] string will be valid only if the bdf_ext is set.

The string memory needs to be null-terminated to avoid the undefined garbage
appended by the subsequent board file name creation.

ath10k_pci 0000:04:00.0: failed to fetch board data for
"bus=pci,vendor=168c,device=003e,subsystem-vendor=168c,subsystem-device=3363��P�����"
from ath10k/QCA6174/hw3.0/board-2.bin

Fixes: f2593cb1 ("ath10k: Search SMBIOS for OEM board file extension")
Reported-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Ryan Hsu <ryanhsu@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>