thunderbolt: Make enum tb_drom_entry_type unsigned

Force enum tb_drom_entry_type to unsigned to fix the following error:

drivers/thunderbolt/eeprom.c:202:39: error: dubious one-bit signed bitfield

Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>