HYPPO dos_chdir trap doesn't set return value correctly.

[gardners]

Test Environment (required)

Any version of MEGA65 core and HYPPO.

Describe the bug

The trap_dos_chdir() trap in HYPPO doesn't set the value of hypervisor_A to $00 on success.
As a result, a successful call to trap_dos_chdir returns $0C instead of $00.

This means that the HYPER trap returns the trap minor number (0x3C)

LDA #$0C
STA $D640
CLV

-> A will contain $0C instead of $00 if the call succeeded.

Expected behavior

A should contain $00 on success

Screenshots
N/A

Additional context
Discovered while working on telephony software, but it will affect any program using this HYPER call.