ath11k: don't call ath11k_pci_set_l1ss for WCN6855

For QCA6390, one PCI related clock drifts sometimes, and it makes
PCI link difficult to quit L1ss. Current implementation fixed this
by configuring some related regs using ath11k_pci_fix_l1ss.

WCN6855 does not have this clock drift problem, so no need to set
these regs.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-01720.1-QCAHSPSWPL_V1_V2_SILICONZ_LITE-1
Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Baochen Qiang <bqiang@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210511162214.29475-8-jouni@codeaurora.org