parisc: don't bother looping in do_signal()

entry.S code had been looping until no pending signals are left
since 2005 anyway; no need to bother with that in do_signal()
itself.  If the failure to set a sigframe up raises SIGSEGV,
we'll just pick it up the next time around the loop(s) in entry.S
anyway.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>