wil6210: fix buffer overflow in wil_txdesc_debugfs_show()

Wrong index comparison logic, found by smatch:

drivers/net/wireless/ath/wil6210/debugfs.c:402 wil_txdesc_debugfs_show() warn: buffer overflow 'wil->vring_tx' 24 <= 24

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>