staging: comedi: ni_pcimio: remove unused variable 'serial_number'

The struct 'ni_private' holds the variable 'serial_number' which post
assignment is never used. Remove the variable and code pertaining to
obtaining its value.

As a side note, this also fixes the following sparse error:
drivers/staging/comedi/drivers//ni_pcimio.c:1229:32: warning: incorrect type in assignment (different base types)
drivers/staging/comedi/drivers//ni_pcimio.c:1229:32:    expected restricted __be32 [usertype] serial_number
drivers/staging/comedi/drivers//ni_pcimio.c:1229:32:    got unsigned int

Signed-off-by: Karthik Nayak <Karthik.188@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>