USB: serial: mos7840: fix crash on resume

In the Linux kernel, the following vulnerability has been resolved:

USB: serial: mos7840: fix crash on resume

Since commit c49cfa917025 ("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.

[ johan: analyse crash and rewrite commit message; set busy flag on

resume; drop bulk-in check; drop unnecessary usb_kill_urb() ]