Fix uninitialized value warnings in CDC

[earlephilhower]

C++ doesn't prefill all values to 0 when only one value is specified in a struct initializer. GCC with -Wall will warn about this if it sees it.

Replace the partial struct initialization with a C++ 0-init {} followed by a manual setting of the CDC RTS/DTR member.

Fixes the -Werror shown below with -Wall

/home/runner/arduino_ide/hardware/pico/rp2040/libraries/Adafruit_TinyUSB_Arduino/src/arduino/../class/cdc/cdc_host.h: In function 'bool tuh_cdc_set_dtr(uint8_t, bool, tuh_xfer_cb_t, uintptr_t)':
/home/runner/arduino_ide/hardware/pico/rp2040/libraries/Adafruit_TinyUSB_Arduino/src/arduino/../class/cdc/cdc_host.h:154:58: error: missing initializer for member 'cdc_line_control_state_t::<unnamed struct>::rts' [-Werror=missing-field-initializers]
  154 |   cdc_line_control_state_t line_state = {.dtr = dtr_state};
      |                                                          ^
/home/runner/arduino_ide/hardware/pico/rp2040/libraries/Adafruit_TinyUSB_Arduino/src/arduino/../class/cdc/cdc_host.h: In function 'bool tuh_cdc_set_rts(uint8_t, bool, tuh_xfer_cb_t, uintptr_t)':
/home/runner/arduino_ide/hardware/pico/rp2040/libraries/Adafruit_TinyUSB_Arduino/src/arduino/../class/cdc/cdc_host.h:162:58: error: missing initializer for member 'cdc_line_control_state_t::<unnamed struct>::dtr' [-Werror=missing-field-initializers]
  162 |   cdc_line_control_state_t line_state = {.rts = rts_state};
      |                                                          ^
cc1plus: all warnings being treated as errors

[earlephilhower]

Failing build here: https://github.com/earlephilhower/arduino-pico/actions/runs/28824125955/job/85482867091?pr=3469