driver/linuxspidev: Clear queue on allocation

driver/linuxspidev: Clear queue on allocation

SWD idle clocks are added to the queue by advancing the queue index
assuming the queue is zeroed. If the queue isn't zeroed, these idle
clocks end up being filled with junk data. Lets clear the queue and
associated buffers on queue allocation.

TEST: Connects successfully and ran the following TCL command:
  dump_image /dev/null 0x20000000 0x42000
  Host: Unnamed Qualcomm SoC with QUPv3 based SPI port
  Target: RT500

Signed-off-by: Richard Pasek <rpasek@google.com>
Change-Id: Ie660c10c27c4d0937ab0629138935ddbf5aeb0ae
Fixes: 83e0293f7ba3 ("Add Linux SPI device SWD adapter support")
Reviewed-on: https://review.openocd.org/c/openocd/+/8730
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Jonathon Reinhart <jrreinhart@google.com>
Tested-by: jenkins