usb: gadget: Introduce free_usb_request helper

Introduce the free_usb_request() function that frees both the request's
buffer and the request itself.

This function serves as the cleanup callback for DEFINE_FREE() to enable
automatic, scope-based cleanup for usb_request pointers.

Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://lore.kernel.org/r/20250916-ready-v1-2-4997bf277548@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250916-ready-v1-2-4997bf277548@google.com