spi: add missing pci_dev_put() before return

pci_get_slot() increases its reference count, the caller must
decrement the reference count by calling pci_dev_put()

Signed-off-by: Wang Qing <wangqing@vivo.com>
Link: https://lore.kernel.org/r/1644890407-65167-1-git-send-email-wangqing@vivo.com
Signed-off-by: Mark Brown <broonie@kernel.org>