prism54: remove questionable down_interruptible usage

Reviewing the semaphore usage I noticed these down_interruptible calls.  Most
of these aren't returning anything, so a caller can't tell if the operation
completed or not.  prism54_wpa_bss_ie_get() returns zero, but it's treated as
the function failing which doesn't seem correct.

Signed-off-by: Daniel Walker <dwalker@mvista.com>
Cc: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>