Simplify generator casting in generateUnicastIpv4PingOffload()

Simplified the code in generateUnicastIpv4PingOffload() by directly
casting the generator to ApfV6GeneratorBase<?>. This is safe because
shouldEnableIpv4PingOffload() already verifies that the ApfV6Generator
is supported. This change improves code readability and reduces
unnecessary checks.

Test: TH
Change-Id: I76e06e1361607e5e2e33df72d0e36df3f9cfae7d
1 file changed