checkpatch: increase the max indentation level

OpenOCD uses longer lines (120 char vs 100) and smaller tab size
(4 char vs 8) wrt Linux kernel coding style.
Clearly deep level of indentation is bad for code readability,
but let's be more permissive on the indentation level.

Change-Id: I16cf0b761145ec6072509dc26bb09c693e89e608
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6167
Tested-by: jenkins