usb: mdm_bridge: Fix race between disconnect and I/O completion handler

There is a possibility of race between device disconnect and I/O
completion handler. In this case disconnect frees bridge device
memory in thread context and I/O completion handler uses already
freed bridge device memory in interrupt context. Hence allocate
bridge device memory at the time of driver load and free it
during unload. Also, do not anchor interrupt and read urb and
unlink them synchronously to avoid accessing transfer buffers
associated to these urbs when driver disconnect frees them.

Guard the race condition by:-
1) Adding check for usb device disconnect state in the I/O
completion handler to avoid further processing I/O completion.

2) Unlinking tx urbs synchronously since asynchronous unlinking
results in the same race condition.

CRs-Fixed: 447591
Change-Id: Iae4c2a1920c16fcf3e1728144a7cfe6ca66c7705
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>