dahdi: Only disable/enable interrupts once when iterating through channels.

dahdi_receive, dahdi_transmit, and dahdi_ec_span are mostly called from
interrupt context anyway, so we can save a few cycles by not saving and
restoring the interrupt flags for every channel.

On one 2.40GHz Xeon test machine, for a span with 24 channels w/o echocan
enabled with ~10000 samples:

Function        Avg Before   Avg After
======================================
dahdi_receive   2.109 us     1.547 us
dahdi_transmit  3.203 us     2.766 us
dahdi_ec_span   0.416 us     0.454 us

NOTE: The time went up slightly on dahdi_ec_span since I did not have
software echocan enabled and this change calls local_irq_save regardless
in dahdi_ec_span.  The slight increase in processing time in this case
is overshadowed by the savings in dahdi_receive and dahdi_transmit. If
echocan was enabled on all the channels there would be a time savings
in that dahdi_ec_span too.

When dahdi_receive/dahdi_transmit are called every millisecond (when
DAHDI_CHUNKSIZE == 8) this saves ~0.1% CPU time for each span.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Acked-by: Kinsey Moore <kmoore@digium.com>
Acked-by: Russ Meyerriecks <rmeyerriecks@digium.com>
Acked-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
Review: https://reviewboard.asterisk.org/r/940/

git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/trunk@9406 a0bf4364-ded3-4de4-8d8a-66a801d63aff

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