drivers: slip: Add more comments for slip write scenario

No functionality changes. Added more comments and used switch cases
for more readability.

Change-Id: I9396270d7368d9b0c923a88f90b44129a1d69cbc
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>
1 file changed