ath11k: use ATH11K_PCI_IRQ_DP_OFFSET for DP IRQ

Like ATH11K_PCI_IRQ_CE0_OFFSET, define ATH11K_PCI_IRQ_DP_OFFSET for
DP to save the IRQ instead of base_vector from MSI config.

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>
Signed-off-by: Baochen Qiang <bqiang@codeaurora.org>
Link: https://lore.kernel.org/r/20211026041655.5112-1-bqiang@codeaurora.org