usb: host: oxu210hp-hcd: fix gcc warning

usb: host: oxu210hp-hcd: fix gcc warning

gcc -O3 warns about correct code:

    inlined from 'oxu_hub_control.constprop' at drivers/usb/host/oxu210hp-hcd.c:3652:3:
include/linux/string.h:411:9: error: argument 1 null where non-null expected [-Werror=nonnull]
  return __builtin_memset(p, c, size);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/usb/host/oxu210hp-hcd.c: In function 'oxu_hub_control.constprop':
include/linux/string.h:411:9: note: in a call to built-in function '__builtin_memset'

Expand the code slightly to let gcc better understand it and
not warn any more.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200107214354.1008937-1-arnd@arndb.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>