Input: wacom - fix runtime PM related deadlock

When runtime PM is enabled by default for input devices, X hangs in
wacom open:
[<ffffffff814a00ea>] mutex_lock+0x1a/0x40
[<ffffffffa02bc94b>] wacom_resume+0x3b/0x90 [wacom]
[<ffffffff81327a32>] usb_resume_interface+0xd2/0x190
[<ffffffff81327b5d>] usb_resume_both+0x6d/0x110
[<ffffffff81327c24>] usb_runtime_resume+0x24/0x40
[<ffffffff8130a2cf>] __pm_runtime_resume+0x26f/0x450
[<ffffffff8130a23a>] __pm_runtime_resume+0x1da/0x450
[<ffffffff8130a53a>] pm_runtime_resume+0x2a/0x50
[<ffffffff81328176>] usb_autopm_get_interface+0x26/0x60
[<ffffffffa02bc626>] wacom_open+0x36/0x90 [wacom]

wacom_open() takes wacom->lock and calls usb_autopm_get_interface(),
which in turn calls wacom_resume() which tries to acquire the lock
again.

The fix is to call usb_autopm_get_interface() first, before we take
the lock.

Since we do not do usb_autopm_put_interface() until wacom_close()
is called runtime PM is effectively disabled for the driver, however
changing it now would risk regressions so the complete fix will
have to wait till the next merge window.

Reported-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>