USB: serial: mos7840: fix crash on resume

Since commit c49cfa91

 ("USB: serial: use generic method if no
alternative is provided in usb serial layer"), USB serial core calls the
generic resume implementation when the driver has not provided one.

This can trigger a crash on resume with mos7840 since support for
multiple read URBs was added back in 2011. Specifically, both port read
URBs are now submitted on resume for open ports, but the context pointer
of the second URB is left set to the core rather than mos7840 port
structure.

Fix this by implementing dedicated suspend and resume functions for
mos7840.

Tested with Delock 87414 USB 2.0 to 4x serial adapter.

Signed-off-by: Dmitry Smirnov <d.smirnov@inbox.lv>
[ johan: analyse crash and rewrite commit message; set busy flag on
         resume; drop bulk-in check; drop unnecessary usb_kill_urb() ]
Fixes: d83b4053

 ("USB: serial: add support for multiple read urbs")
Cc: stable@vger.kernel.org	# 3.3
Signed-off-by: Johan Hovold <johan@kernel.org>