drivers/linuxspidev: fix compilation on Linux older than 3.15

drivers/linuxspidev: fix compilation on Linux older than 3.15

Although the commit [1] which introduced SPI_IOC_WR_MODE32 is 10 years
old, some hardware may enforce old Linux because the vendor
didn't bother with system updates.

Change-Id: I76d0b38c8646b1be329418860916b9f01b90990d
Link: [1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/include/uapi/linux/spi/spidev.h?h=linux-3.15.y&id=dc64d39b54c1e9db97a6fb1ca52598c981728157
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/8759
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>