drivers/cmsis-dap: speed up sending multiple HID requests

drivers/cmsis-dap: speed up sending multiple HID requests

The performance of CMSIS-DAP in long data transfers was improved substantially in
ef02b69b14d133b061217a91add5a028a77e86bc. But it not as good as some
other USB/MCU based adapters. Using HID and therefore interrupt endpoint
is slower than USB bulk transfer.

CMSIS-DAP adapter implements multiple HID buffer handling and OpenOCD already
reads number of buffers from info command.

This change adds capability to sumbit more than one HID requests before
driver waits for a HID response. This scenario is used for long transfers only.
Results show about double speed on USB FS and ~140% speed on USB HS:

                                         | w/o this change | with multi HIDrq
-----------------------------------------+-----------------+-----------------
Open source CMSIS-DAP, USB FS, adapter_khz 1000
dump_image ram32k.bin 0x1fffe000 0x8000  |   23.225 KiB/s  |   45.901 KiB/s
load_image ram32k.bin 0x1fffe000         |   23.324 KiB/s  |   46.552 KiB/s

Cypress' Kitprog in CMSIS-DAP mode, USB FS, adapter_khz 1000 (over firmware limit)
dump_image ram64k.bin 0x20000000 0x10000 |   15.537 KiB/s  |   42.558 KiB/s
load_image ram64k.bin 0x20000000         |   15.605 KiB/s  |   43.291 KiB/s

Atmel's EDBG, USB HS, adapter_khz 10000 (#3945 applied)
dump_image ram384k.bin 0x20400000 0x6000 |  248.402 KiB/s  |  345.250 KiB/s
load_image ram384k.bin 0x20400000        |  256.039 KiB/s  |  365.945 KiB/s

Change-Id: I9edbe018086176d357c6aaba5d6b657a5e5e1c64
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4080
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>