ilitek: Port to Kernel 5.4

Replace deprecated functions vfs_read/vfs_write with
kernel_read/kernel_write.  Use the KERNEL_DS define instead of the
deprecated get_de() function.

Replace the deprecated do_gettimeofday() function call with the recent
ktime_get_real_ts64() function and move from struct timeval to struct
timespec64 to do this.

Fix a compiler error that complains about large frames by reducing the
stack size in the function ilitek_sensortest_write().