reg_fp* values are wrong when PULP_Zfinx is enabled

Created by: abukharmeh

https://github.com/openhwgroup/cv32e40p/blob/b7c91867b2f611013dbe98150e745b1a2ff6b7c6/rtl/cv32e40p_decoder.sv#L1412-L1414

The values of reg_fp_* are assigned incorrectly when Zfinx is enabled, and then overridden in the Register file, and now in #683 in the tracer.

In my opinion, this makes the design less readable for no apparent reason, I think PULP_ZFINX parameter value should be passed to the decoder module, and these values should be assigned to !PULP_ZFINX

Perhaps one would also want to add an assertion that these values are never high when Zfinx is enabled