The iwx(4) binding command can fail with 5 Ghz channels.
Some firmware versions don't expect LMAC_5G_INDEX in the binding command.
I have no idea what "CDB" stands for, but this check matches what the
Linux driver does and makes the command work on -48 firmware.
The Linux driver actually checks two capabilities, CAPA_BINDING_CDB_SUPPORT
and CAPA_CDB_SUPPORT. Because BINDING_CDB_SUPPORT is set for all firmware
versions used by iwx(4) we don't need to check it explicitly. Linux has to
check both flags because their code runs with iwm(4) and iwx(4) devices.