ath10k: Modify CE4 src buffer entries to 2048 for WCN3990

CE4 is host to target HTT tx pipe, tx completion are not served
on time when CPU is loaded and this cause ce src ring full condition
due to less no of src buffer entries.

To mitigate the issue increase CE4 src buffer entries to 2048.

Testing:
        Tested on QCS404 platform(WCN3990 HW)
        Tested FW: WLAN.HL.3.1-00784-QCAHLSWMTPLZ-1

Signed-off-by: Govind Singh <govinds@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>