drivers: rmnet_perf: Lower TCP and UDP coalescing byte limit

Lowering the byte limit from 65535 to 65000 helps prevent the truesize of
the final coalesced SKB from growing into the next power of 2 (~131,000).
This helps reduce the memory footprint of the module.

Change-Id: Ide9bbf2a899e29b1fa23072cee4bffba6b0ff65b
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>