staging: comedi: ni_labpc: unlock spinlock on command error

When starting the acquisition the labpc_ai_cmd function was
exiting without unlocking the spinlock. This results in a
sparse warning:

warning: context imbalance in 'labpc_ai_cmd' - different lock contexts for basic block

Add the missing spin_unlock_irqrestore calls.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>