zebra: Fixes allowing SRv6 func-bits length 0

zebra: Fixes allowing SRv6 func-bits length 0

In current code, when a user inputs function length as 0 for a locator,
the code treats it as a "not configured" value and assigned default (16)
value to it.

This is wrong since the locator default is only to be used when the
function-bits (0-64) is not specified in the locator prefix config.

Fix is to use default value only when the user has not specified the
func-bits

Signed-off-by: Rajasekar Raja <rajasekarr@nvidia.com>