zswap: Uncross module parameter setting functions

The "compressor" and "zpool" parameters of the zswap module each has
a custom setter function that calls __zswap_param_set with specific
parameters, but the "zpool" setter uses parameters that are correct for
the "compressor" parameter, and vice-versa.

Fix this by swapping the function bodies over.

Fixes: 90b0fc26 ("zswap: change zpool/compressor at runtime")

Signed-off-by: Phil Elwell <phil@raspberrypi.com>