usb_storage.c: initialize device type

The device type was left uninitialized which caused later tests
against DEV_TYPE_UNKNOWN to fail. In the result, "usb part" would
attempt to print information about non-existent devices like this:

	=> usb part
	print_part of 0

	Partition Map for USB device 0  --   Partition Type: DOS

	Partition     Start Sector     Num Sectors     Type
	    1                    0         2031616      f8

	print_part of 1
	## Unknown partition table

	print_part of 2
	## Unknown partition table

	print_part of 3
	## Unknown partition table

	print_part of 4
	## Unknown partition table
	=>

By initializing the type as DEV_TYPE_UNKNOWN we avoid all the
"Unknown partition table" messages.

[Note: the "print_part of ?" messages is left over debug code that
will be removed in another patch.]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Remy Bohmer <linux@bohmer.net>