usb: misc: brcmstb-usb-pinmap: check return value after calling platform_get_resource()

It will cause null-ptr-deref if platform_get_resource() returns NULL,
we need check the return value.

Fixes: 517c4c44 ("usb: Add driver to allow any GPIO to be used for 7211 USB signals")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20210605080914.2057758-1-yangyingliang@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>