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() ]