ath11k: Fix uninitialized symbol 'rx_buf_sz'

Add missing else statement in ath11k_dp_rx_process_mon_status()
to fix below smatch warnings,
    drivers/net/wireless/ath/ath11k/dp_rx.c:3105
    ath11k_dp_rx_process_mon_status()
    error: uninitialized symbol 'rx_buf_sz'.

Fixes: ab18e3bc ("ath11k: Fix pktlog lite rx events")

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Anilkumar Kolli <quic_akolli@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/1642605793-13518-1-git-send-email-quic_akolli@quicinc.com