USB: make transfer_buffer_lengths in struct urb field u32

Roel Kluin pointed out that transfer_buffer_lengths in struct urb was
declared as an 'int'.  This patch changes this field to be 'u32' to
prevent any potential negative conversion and comparison errors.

This triggered a few compiler warning messages when these fields were
being used with the min macro, so they have also been fixed up in this
patch.

Cc: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>