ath10k: replace config_enabled() with IS_REACHABLE()

Commit 97f2645f ("tree-wide: replace config_enabled() with
IS_ENABLED()") mostly did away with config_enabled().

This is one of the postponed TODO items as config_enabled() is used
for a tristate option here.  Theoretically, config_enabled() is
equivalent to IS_BUILTIN(), but I guess IS_REACHABLE() is the best
fit for this case because both CONFIG_HWMON and CONFIG_ATH10K are
tristate.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>