usb: misc: brcmstb-usb-pinmap: Fix an IS_ERR() vs NULL check

The devm_ioremap() function doesn't return error pointers, it returns
NULL on error.

Fixes: 517c4c44

 ("usb: Add driver to allow any GPIO to be used for 7211 USB signals")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20201102075655.GA4163205@mwanda
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>