Fix -Warray-bounds warning in drivers/sbus/char/openprom.c

[GustavoARSilva]

Found after building (with GCC-13) sparc with sparc64_defconfig:

In function 'opromgetprop',
    inlined from 'openprom_sunos_ioctl.isra' at drivers/sbus/char/openprom.c:312:11:
drivers/sbus/char/openprom.c:141:24: warning: array subscript 1 is above array bounds of 'char[1]' [-Warray-bounds=]
  141 |         op->oprom_array[len] = '\0';
      |         ~~~~~~~~~~~~~~~^~~~~
In file included from drivers/sbus/char/openprom.c:31:
arch/sparc/include/uapi/asm/openpromio.h: In function 'openprom_sunos_ioctl.isra':
arch/sparc/include/uapi/asm/openpromio.h:16:17: note: while referencing 'oprom_array'
   16 |         char    oprom_array[1];         /* Holds property names and values. */
      |                 ^~~~~~~~~~~

[kees]

Fixed in commit 221a4b56ec70c1a9f09ce78751613d16c508a4ce.