Fix the (i)Stallion driver's putchar() and break_ctl() ops

Fix the Stallion driver's putchar() and break_ctl() ops and iStallion's
putchar() to return values.

Is it actually possible for putchar() or break_ctl() to be called with tty ==
NULL or can the check be discarded?

Should stl_write() be returning 0 if tty->driver_data is NULL or tx.buf is
NULL?  Is this even possible?

I've made Stallion's functions return -EINVAL as stli_breakctl() if the checks
fail.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>