USB: serial: keyspan: remove redundant assignment to pointer data

The pointer data is being assigned a value that is not being
read afterwards, it is being re-assigned later inside a do-while
loop. The assignment is redundant and can be removed.

Cleans up clang scan warning:
drivers/usb/serial/keyspan.c:924:2: warning: Value stored to 'data'
is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>