spi: spi-loopback-test: Fix formatting issues in function header blocks

spi: spi-loopback-test: Fix formatting issues in function header blocks

Kerneldoc function parameter descriptions must be in '@.*: ' format.

Fixes the following W=1 kernel build warning(s):

 drivers/spi/spi-loopback-test.c:897: warning: Function parameter or member 'spi' not described in 'spi_test_execute_msg'
 drivers/spi/spi-loopback-test.c:897: warning: Function parameter or member 'test' not described in 'spi_test_execute_msg'
 drivers/spi/spi-loopback-test.c:897: warning: Function parameter or member 'tx' not described in 'spi_test_execute_msg'
 drivers/spi/spi-loopback-test.c:897: warning: Function parameter or member 'rx' not described in 'spi_test_execute_msg'
 drivers/spi/spi-loopback-test.c:970: warning: Function parameter or member 'spi' not described in 'spi_test_run_test'
 drivers/spi/spi-loopback-test.c:970: warning: Function parameter or member 'test' not described in 'spi_test_run_test'
 drivers/spi/spi-loopback-test.c:970: warning: Function parameter or member 'tx' not described in 'spi_test_run_test'
 drivers/spi/spi-loopback-test.c:970: warning: Function parameter or member 'rx' not described in 'spi_test_run_test'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Martin Sperl <kernel@martin.sperl.org>
Cc: linux-spi@vger.kernel.org
Link: https://lore.kernel.org/r/20200717135424.2442271-2-lee.jones@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>