stlink-v1: code cleanup

stlink-v1: code cleanup

This patch moves the bulk of the stlink read/write code into the
stlink_usb_xfer set of functions and implements stlink_usb_recv
in terms of the generic stlink_usb_xfer

stlink_usb_xfer will be needed to implement stlink_usb_send
without code duplication

stlink_usb_xfer:
-sends the stlink command
-performs a read or write (as requested)
-checks the status (v1 only)

Change-Id: I0137d52620bd4883d46c9977a9e73f67622000a1
Signed-off-by: Szymon Modzelewski <szmodzelewski@gmail.com>
Reviewed-on: http://openocd.zylin.com/477
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>