ath10k: fix potential issue of peer stats allocation

STA number was not restored if OOM happened.

Tested: QCA9984 with firmware ver 10.4-3.9.0.1-00018
Signed-off-by: Zhi Chen <zhichen@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>