qcacld-3.0: Fix compiler issue about cnss_set_pcie_gen_speed

Since some platform driver does not support for PCIE gen switch,
Using macro PCIE_GEN_SWITCH to control whether function
cnss_set_pcie_gen_speed is invoke or not.

Change-Id: I478cd6426b005014e85c0b3c1c31963045d7322b
CRs-Fixed: 2767110

Kbuild

@@ -2894,6 +2894,9 @@ cppflags-$(CONFIG_HL_DP_SUPPORT) += -DQCA_COMPUTE_TX_DELAY_PER_TID
 cppflags-$(CONFIG_LL_DP_SUPPORT) += -DCONFIG_LL_DP_SUPPORT
 cppflags-$(CONFIG_LL_DP_SUPPORT) += -DWLAN_FULL_REORDER_OFFLOAD
 
+# For PCIe GEN switch
+cppflags-$(CONFIG_PCIE_GEN_SWITCH) += -DPCIE_GEN_SWITCH
+
 # For OOB testing
 cppflags-$(CONFIG_WLAN_FEATURE_WOW_PULSE) += -DWLAN_FEATURE_WOW_PULSE