tests: spi: correct a spi buffer length issue

Make the buffer length for tx and rx are the same for
spi_transceive API call. QMSI only supports equal length.
This is clearly specifed in spi header file.

Also correct some coding style issues.

Change-Id: Ifd34683e8813dae9b692ad453176a93cb3848427
Signed-off-by: Baohong Liu <baohong.liu@intel.com>