app: tests: Fixed SPI test compile error

Fixed SPI test compile error.

app/tests/spi_test.c:50:2: error: implicit declaration of function
'gpio_set'.
app/tests/spi_test.c:51:2: error: pointer targets in passing argument 2
of 'spi_qup_write' differ in signedness.
app/tests/spi_test.c:77:15: error: comparison between signed and
unsigned integer expressions.
app/tests/spi_test.c:63:15: error: unused variable 'max_speed_hz'.
app/tests/spi_test.c:114:2: error: pointer targets in passing argument 2
of 'spidev_read_cmd' differ in signedness.

Change-Id: If22e54cae31670babb067e8ef1e5aa46e07caaa1