spi: fix counting in spi-loopback-test code

These variables are always used uninitialized:

drivers/spi/spi-loopback-test.c: In function 'spi_test_run_iter':
drivers/spi/spi-loopback-test.c:768:17: warning: 'rx_count' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/spi/spi-loopback-test.c:762:17: warning: 'tx_count' may be used uninitialized in this function [-Wmaybe-uninitialized]

Adding an explicit initialization seems to be the only
workable solution here, to make the code behave correctly
and build without warning.

Fixes: 84e0c4e5 ("spi: add loopback test driver to allow for spi_master regression tests")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@kernel.org>