sparc32: make __cmpxchg_u32() return u32

Conversion between u32 and unsigned long is tautological there,
and the only use of return value is to return it from
__cmpxchg() (which return unsigned long).

Get rid of explicit casts in __cmpxchg_u32() call, while we are
at it - normal conversions for arguments will do just fine.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>