parisc: __cmpxchg_u32(): lift conversion into the callers

__cmpxchg_u32() return value is unsigned int explicitly cast to
unsigned long.  Both callers are returns from functions that
return unsigned long; might as well have __cmpxchg_u32()
return that unsigned int (aka u32) and let the callers convert
implicitly.

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