rust: pwm: Fix potential memory leak on init error

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().

Fixes: 7b3dce814a15 ("rust: pwm: Add Kconfig and basic data structures")
Signed-off-by: Kari Argillander <kari.argillander@gmail.com>
Acked-by: Michal Wilczynski <m.wilczynski@samsung.com>
Link: https://patch.msgid.link/20260102-pwm-rust-v2-1-2702ce57d571@gmail.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>