[rtl] fix natural condition

[NikLeberg]

Commit #41eec5a introduced an error for me when synthesizing with Quartus or simulating with ModelSim/QuestaSim.

QuestaSim:

# ** Error: ../lib/neorv32/rtl/core/neorv32_cpu_control.vhd(136): (vopt-1144) Value -32 (of type std.STANDARD.NATURAL) is out of range 0 to 2147483647.

Quartus:

Error (10528): VHDL error at neorv32_cpu_control.vhd(136): value "-32" is outside the target constraint range (0 to 2147483647) File: /workspaces/neorv32_soc/lib/neorv32/rtl/core/neorv32_cpu_control.vhd Line: 136
Error (10346): VHDL error at neorv32_package.vhd(1081): formal port or parameter "val_t" must have actual or default value File: /workspaces/neorv32_soc/lib/neorv32/rtl/core/neorv32_package.vhd Line: 1081
Error (10657): VHDL Subprogram error at neorv32_cpu_control.vhd(136):  failed to elaborate call to subprogram "cond_sel_natural_f" File: /workspaces/neorv32_soc/lib/neorv32/rtl/core/neorv32_cpu_control.vhd Line: 136
Error (12152): Can't elaborate user hierarchy "neorv32_cpu_smp:neorv32_cpu_smp_inst|neorv32_cpu:\neorv32_cpu_gen:0:neorv32_cpu_inst|neorv32_cpu_control:neorv32_cpu_control_inst" File: /workspaces/neorv32_soc/lib/neorv32/rtl/core/neorv32_cpu.vhd Line: 198

More specifically in neorv32_cpu_control.vhd#L136 the literal HPM_CNT_WIDTH-32 evaluates to -32 if no HPMs are enabled and is no longer a valid natural type.

I've added a quick fix for this by introducing a cond_sel_int_f function (hasn't that been there some time ago 🤔?). Fix works for me.

[stnolting]

Thanks for fixing!