[POWERPC] mpic_u3msi: Failed allocation unnoticed

bitmap_find_free_region(), called by mpic_msi_alloc_hwirqs() may return
signed, but hwirq is unsigned.  A failed allocation remains unnoticed.

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Paul Mackerras <paulus@samba.org>