USB: ccid_bridge: Provide bulk message write/read stats via debugfs

The bulk message timeouts are observed occasionally during mass storage
transfers. This debug facility helps in identifying such timeout issues.

The sample output of the stats debugfs file.

ccid_bridge: connected
ccid_bridge: opened
total writes: 444
total reads: 444
write/read timeout val: 1000
write_timeout: 0
read_timeout: 0
write_max_time (msec): 130
read_max_time: (msec): 857

CRs-Fixed: 659909
Change-Id: I9787288c10d23b176c9316feed82d41bd2adae65
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>