spi: spidev_test: Fix input file check when transferring file

Check the input file fd instead of spidev fd.
The spidev fd is supposed to be OK otherwise the transfer_file() function
would not be called at all.

Signed-off-by: Michal Vokáč <vokac.m@gmail.com>
Reviewed-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>