usb: typec: avoid format-overflow warning

gcc-8 points out that the fix-byte buffer might be too small if
desc->mode is a three-digit number:

drivers/usb/typec/class.c: In function 'typec_register_altmode':
drivers/usb/typec/class.c:502:32: error: '%d' directive writing between 1 and 3 bytes into a region of size 2 [-Werror=format-overflow=]
  sprintf(alt->group_name, "mode%d", desc->mode);
                                ^~
drivers/usb/typec/class.c:502:27: note: directive argument in the range [0, 255]
  sprintf(alt->group_name, "mode%d", desc->mode);
                           ^~~~~~~~
drivers/usb/typec/class.c:502:2: note: 'sprintf' output between 6 and 8 bytes into a destination of size 6
  sprintf(alt->group_name, "mode%d", desc->mode);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I assume this cannot happen in practice, but we can simply make the
string long enough to avoid the warning. This uses the two padding
bytes that already exist after the string.

Fixes: 4ab8c18d ("usb: typec: Register a device for every mode")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>