rust: pwm: Fix potential memory leak on init error

In the Linux kernel, the following vulnerability has been resolved: rust: pwm: Fix potential memory leak on init error When initializing a PWM chip using pwmchip_alloc(), the allocated device owns an initial reference that must be released on all error paths. If __pinned_init() were to fail, the allocated pwm_chip would currently leak because the error path returns without calling pwmchip_put().