net: af_mpls.c add space before open parenthesis

Adding space after switch keyword before open
parenthesis for readability purpose.

This patch fixes the checkpatch.pl warning:
space required before the open parenthesis '('

Signed-off-by: Suraj Deshmukh <surajssd009005@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>