ath11k: set WMI pipe credit to 1 for QCA6390

For QCA6390, set wmi credit to 1 to avoid back-to-back write to
shadow register when shadow register is enabled.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1601544890-13450-4-git-send-email-kvalo@codeaurora.org