wer to use AR5K_SLEEP_CTL_SLE_ALLOW for Network sleep
 * Preserve sleep duration field while setting AR5K_SLEEP_CTL
   and reduce delays & checks for register's status (got this from
   decompiling & dumps, it works for me but it needs testing)

Changes-licensed-under: ISC
Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
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