USB: make actual_length in struct urb field u32

actual_length should also be a u32 and not a signed value.  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>