sparc: explicitly cast negative phandle checks to s32

When we switched sparc from using 'int's to 'phandle's (which is a u32), we
neglected to do anything with the various checks for -1.  For those tests,
explicitly cast the phandles to s32.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>