ath10k: increase max client to 512 in qca99x0

When max client was set to 512 in qca99x0, there was host memory
alloc failure during wmi service ready event handling. This issue
got resolved now, increasing max client limit from 256 to 512.

Signed-off-by: Raja Mani <rmani@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>