xen: make use of xenbus_read_unsigned() in xen-tpmfront

Use xenbus_read_unsigned() instead of xenbus_scanf() when possible.
This requires to change the type of one read from int to unsigned,
but this case has been wrong before: negative values are not allowed
for the modified case.

Cc: peterhuewe@gmx.de
Cc: tpmdd@selhorst.net
Cc: jarkko.sakkinen@linux.intel.com
Cc: jgunthorpe@obsidianresearch.com
Cc: tpmdd-devel@lists.sourceforge.net

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>