firewire: extract shared helpers from fwe and fwip

Add fw_net.h with common inline helpers used by both if_fwe and
if_fwip: ISO chunk init, TX xfer allocation, xferlist free,
send queue drain, and D

firewire: extract shared helpers from fwe and fwip

Add fw_net.h with common inline helpers used by both if_fwe and
if_fwip: ISO chunk init, TX xfer allocation, xferlist free,
send queue drain, and DEVICE_POLLING ioctl handling.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D57615

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