ipfw: use unsigned int for dummynet bandwidth

This allows the maximum value of 4294967295 (~4Gb/s) instead of previous
value of 2147483647 (~2Gb/s).

Reviewed by: np, scottl
Obtained from: pfSense
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31582