package/fwts: use ifeq instead of ifdef

The conditional code using ifdef was added in 2018 and works as
expected.
But there is no reason to use ifdef instead of ifeq.

For consistence, switch to use ifeq like almost all packages already do.

Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Erico Nunes <nunes.erico@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>