ath10k: Fix DMA burst size

A value of zero indicates that 128B is the maximum
DMA request size for read/writes. But PCI cards based
on AR9880 can support 256B, so enable this for
the 10.2 firmware.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>