swmod usage on field inside read only register

Trying to activate swmod option on a read only field generate incorrect verilog code if within a register fully read only.

The register *_swwr signal is missing and not declared.

(vlog-2730) Undefined variable: 'regname_f_fieldname_swwr_condition'.

This is minor issue as it does not make much sense to have write action on readonly status register. It has been found where a status register try to implement a wclr functionality outside rif, in the user rtl. May be a strange but valid scenario. So would need a fix of pyrift.