rmnet_perf: Cast expected packets for dlhdr

Fixes issue which was caused by modem over
reporting the number of packets that a burst
contains. This resulted in bootup sending
packets which tainted the number of expected
packets since it was being interpreted as an
large unsigned int instead of a a small
negative number.

Change-Id: I31ec6265d6cb684c63fb12916a5ef073fa3c9605
Singed-off-by: Conner Huff <chuff@codeaurora.org>