mptcp: remove unnecessary else statements

The 'else' statements are not needed here, because their previous 'if'
block ends with a 'return'.

This fixes CheckPatch warnings:

  WARNING: else is not generally useful after a break or return

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20240514011335.176158-7-martineau@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>