drivers: input: input_gt911: fix OOB buffer write to touch points array

GT911 driver was reading an additional touch point into the stack
defined touch point array, causing an out-of-bounds write on the stack.
Fix this issue by adjusting the limit on the for loop, and add a note
to the driver as to why we don't need to validate the number of points
reported for future developers

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>