Improve TX performance of i8254x (PA-GE)

Send up to 16 packets per task sleep interval. Based on approach in dev_mueslix.c. Needs review by someone better acquainted with the code than me.

This greatly improves the throughput of packets being routed, from about 100pps to 1600pps.
Tested with 7200 PA-GE and the default dual FastEthernet (C7200-IO-2FE?)

It looks like many if not all the other network drivers do this already:

~~~
$ grep PASS_COUNT common/*.c
common/dev_am79c971.c:#define AM79C971_TXRING_PASS_COUNT  16
common/dev_am79c971.c:   for(i=0;i<AM79C971_TXRING_PASS_COUNT;i++)
common/dev_dec21140.c:#define DEC21140_TXRING_PASS_COUNT  32
common/dev_dec21140.c:   for(i=0;i<DEC21140_TXRING_PASS_COUNT;i++)
common/dev_mpc860.c:#define MPC860_TXRING_PASS_COUNT  16
common/dev_mpc860.c:   for(i=0;i<MPC860_TXRING_PASS_COUNT;i++)
common/dev_mpc860.c:   for(i=0;i<MPC860_TXRING_PASS_COUNT;i++)
common/dev_mueslix.c:#define MUESLIX_TXRING_PASS_COUNT  16
common/dev_mueslix.c:   for(i=0;i<MUESLIX_TXRING_PASS_COUNT;i++) {
common/dev_pa_a1.c:#define TI1570_TXDMA_PASS_COUNT  16
common/dev_pa_a1.c:   for(i=0;i<TI1570_TXDMA_PASS_COUNT;i++)
~~~