USB: serial: mos7840: fix probe error handling

The driver would return success and leave the port structures
half-initialised if any of the register accesses during probe fails.

This would specifically leave the port control urb unallocated,
something which could trigger a NULL pointer dereference on interrupt
events.

Fortunately the interrupt implementation is completely broken and has
never even been enabled...

Note that the zero-length-enable register write used to set the zle-flag
for all ports is moved to attach.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>