staging: comedi: dt2811: remove redundant initialization of 'ns'

Variable ns is being initialized with a value that is never read, ns
is being re-assigned a new value later on. Remove the redundant
initialization.

Cleans up clang warning:
drivers/staging/comedi/drivers/dt2811.c:310:21: warning: Value stored
to 'ns' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>