net: tftp: fix option validation as per RFCs

RFC2348, RFC2349:
- Option string is case in-sensitive.
- Client must generate ERR pkt in case option value mismatch in server OACK
- Fix debug print for options

Signed-off-by: Ravik Hasija <rahasij@linux.microsoft.com>
Reviewed-By: Ramon Fried <rfried.dev@gmail.com>