Sign comparison warnings when compiling the NI-845x

[martonmiklos]

Issue for myself to fix sometime:

Recently did a build with Mingw and failed unless passing WARNERROR=no to make

ni845x_spi.c: In function 'ni845x_spi_open':
ni845x_spi.c:188:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (!serial || strtol(serial, NULL, 16) == serial_as_number)
                                           ^~
ni845x_spi.c: In function 'usb8452_spi_set_io_voltage':
ni845x_spi.c:271:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (i = 0; i < ARRAY_SIZE(usb8452_io_voltages_in_100mV); i++) {
                 ^
ni845x_spi.c: At top level:
ni845x_spi.c:534:13: warning: initialization from incompatible pointer type [-Wincompatible-pointer-types]
  .command = ni845x_spi_transmit,
             ^~~~~~~~~~~~~~~~~~~
ni845x_spi.c:534:13: note: (near initialization for 'spi_programmer_ni845x.command')
ni845x_spi.c: In function 'ni845x_spi_init':
ni845x_spi.c:559:39: warning: comparison is always false due to limited range of data type [-Wtype-limits]
   if (strlen(CS_str) > 1 || CS_number < 0 || 7 < CS_number) {