Bluetooth: hci_qca: Fix NULL vs IS_ERR_OR_NULL check in qca_serdev_probe

The function devm_gpiod_get_index() return error pointers on error.
Thus devm_gpiod_get_index_optional() could return NULL and error pointers.
The same as devm_gpiod_get_optional() function. Using IS_ERR_OR_NULL()
check to catch error pointers.

Fixes: 77131dfe

 ("Bluetooth: hci_qca: Replace devm_gpiod_get() with devm_gpiod_get_optional()")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>