From dmesg, I found that the firmware for my device couldn't be found and was
lead to believe it was provided by the package above. My system already had it
installed, but the files were not present in /lib/firmware/brcm. I don't know
how my system got into this state and figuring that the installation went wrong
somehow, I decided to try and reinstall with

# zypper in -f bcm20702a1-firmware-1201650-5.1.noarch

This looked like it was successful, the output ending with

Converting BCM_DriverOnly/64/BCM20702A1_001.002.014.1443.1467.hex to
BCM20702A1-0b05-17cb.hcd 
Converting BCM_DriverOnly/64/BCM20702A1_001.002.014.1443.1469.hex to
BCM20702A1-0b05-17cf.hcd 
Converting BCM_DriverOnly/64/BCM20702A1_001.002.014.1483.1647.hex to
BCM20702A1-05ac-21e1.hcd 
Converting BCM_DriverOnly/64/BCM20702A1_001.002.014.1483.1669.hex to
BCM20702A1-0a5c-21e6.hcd 
Converting BCM_DriverOnly/64/BCM20702A1_001.002.014.1443.1467.hex to
BCM20702A1-050d-065a.hcd

but after completion, the firmware files were still not present in
/lib/firmware/brcm. 

Over on the openSUSE forums, someone told me that the installation script
unconditionally removes the files in its post-uninstall step and that that is
always run on update (see the thread here:
https://forums.opensuse.org/showthread.php/542851-Problem-reinstalling-bcm20702a1-firmware-1201650-5-1-noarch?p=2953852#post2953852).