Bluetooth: hci_bcm: Check for error irq

For the possible failure of the platform_get_irq(), the returned irq
could be error number and will finally cause the failure of the
request_irq().
Consider that platform_get_irq() can now in certain cases return
-EPROBE_DEFER, and the consequences of letting request_irq() effectively
convert that into -EINVAL, even at probe time rather than later on.
So it might be better to check just now.

Fixes: 0395ffc1

 ("Bluetooth: hci_bcm: Add PM for BCM devices")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>