staging: comedi: dt2801: fix using uninitialized variable warnings

As pointed out by Fengguang Wu, the calls to dt2801_readdata() return the
data read thru a pointer (the address of a local variable) passed as a
parameter. If this local variable is not initialized it produces a warning
during the build of the form:

drivers/staging/comedi/drivers/dt2801.c: In function 'dt2801_ai_insn_read':
drivers/staging/comedi/drivers/dt2801.c:273:14: warning: 'hb' is used
      uninitialized in this function [-Wuninitialized]

Fix these warning by making sure the local variables are initialized.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>