um: Cleanup syscall_handler_t cast in syscalls_32.h

Like in f4f03f29


"um: Cleanup syscall_handler_t definition/cast, fix warning",
remove the cast to to fix the compiler warning.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Richard Weinberger <richard@nod.at>