net: tcp: switch to new API for k_work_pending

Uses of k_work_pending are to be replaced by k_work_is_pending which
conforms to current proposed naming guidelines.

Both uses in this file are fragile: that a work item is pending does
not mean changes since it was first submitted are guaranteed to be
seen when the work item begins (began) executing.

As long as this module is expected to be replaced by tcp2 it doesn't
seem worth trying to fix the logic, so just switch to the new function
name.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>