ch32: select USBHS device speed from rhport configuration

Honor rh_init->speed when configuring the USBHS controller instead of
selecting speed with TUD_OPT_HIGH_SPEED and rejecting full-speed builds.
Map AUTO/HIGH to high speed, FULL to full speed, and LOW to low speed;
reject unsupported values. This ports the speed-selection part of #3727.

The low-speed register setting does not establish low-speed enumeration
support: the reset-completion path still relies on SOF.

Validation:
- Full example sets passed on NanoCH32V305 and CH32V307: 54 builds.
- NanoCH32V305 uac2_headset builds with USBHS forced to full speed.
- NanoCH32V305 uac2_headset stack size: +44 bytes code, no RAM change.

src/portable/wch/dcd_ch32_usbhs.c

diff --git a/src/portable/wch/dcd_ch32_usbhs.c b/src/portable/wch/dcd_ch32_usbhs.c
index a7c94f5..d531feb 100644
--- a/src/portable/wch/dcd_ch32_usbhs.c
+++ b/src/portable/wch/dcd_ch32_usbhs.c
@@ -141,7 +141,6 @@
 
 bool dcd_init(uint8_t rhport, const tusb_rhport_init_t *rh_init) {
   (void)rhport;
-  (void)rh_init;
 
   memset(&xfer_status, 0, sizeof(xfer_status));
   ep0_tog = true;
@@ -152,12 +151,15 @@
 
   USBHSD->CONTROL = 0;
 
-  #if TUD_OPT_HIGH_SPEED
-  USBHSD->CONTROL = USBHS_DMA_EN | USBHS_INT_BUSY_EN | USBHS_HIGH_SPEED;
-  #else
-    #error OPT_MODE_FULL_SPEED not currently supported on CH32
-  USBHSD->CONTROL = USBHS_DMA_EN | USBHS_INT_BUSY_EN | USBHS_FULL_SPEED;
-  #endif
+  if (rh_init->speed == TUSB_SPEED_HIGH || rh_init->speed == TUSB_SPEED_AUTO) {
+    USBHSD->CONTROL = USBHS_DMA_EN | USBHS_INT_BUSY_EN | USBHS_HIGH_SPEED;
+  } else if (rh_init->speed == TUSB_SPEED_FULL) {
+    USBHSD->CONTROL = USBHS_DMA_EN | USBHS_INT_BUSY_EN | USBHS_FULL_SPEED;
+  } else if (rh_init->speed == TUSB_SPEED_LOW) {
+    USBHSD->CONTROL = USBHS_DMA_EN | USBHS_INT_BUSY_EN | USBHS_LOW_SPEED;
+  } else {
+    return false;
+  }
 
   USBHSD->INT_EN = 0;
   USBHSD->INT_EN = USBHS_SETUP_ACT_EN | USBHS_TRANSFER_EN | USBHS_BUS_RST_EN | USBHS_SUSPEND_EN;