All the info we need to pass to transport function can be captured in
a struct us_blk_dev and two byte arrays, so having a dedicated struct
with many unused fields doesn't really buy us anything. Drop the
struct and convert the rest of the code to pass needed data
explicitly.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>