ath10k: fix build error when hwmon is off

kbuild reported a linking error:

ERROR: "devm_hwmon_device_register_with_groups"
[drivers/net/wireless/ath/ath10k/ath10k_core.ko] undefined!

Fix it by returning early and letting the compiler to optimise out the function
call.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>