platform/x86: hp-bioscfg: Fix kernel panic in GET_INSTANCE_ID macro

platform/x86: hp-bioscfg: Fix kernel panic in GET_INSTANCE_ID macro

The GET_INSTANCE_ID macro that caused a kernel panic when accessing sysfs
attributes:

1. Off-by-one error: The loop condition used '<=' instead of '<',
   causing access beyond array bounds. Since array indices are 0-based
   and go from 0 to instances_count-1, the loop should use '<'.

2. Missing NULL check: The code dereferenced attr_name_kobj->name
   without checking if attr_name_kobj was NULL, causing a null pointer
   dereference in min_length_show() and other attribute show functions.

The panic occurred when fwupd tried to read BIOS configuration attributes:

  Oops: general protection fault [#1] SMP KASAN NOPTI
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: 0010:min_length_show+0xcf/0x1d0 [hp_bioscfg]

Add a NULL check for attr_name_kobj before dereferencing and corrects
the loop boundary to match the pattern used elsewhere in the driver.

Cc: stable@vger.kernel.org
Fixes: 5f94f181ca25 ("platform/x86: hp-bioscfg: bioscfg-h")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patch.msgid.link/20260115203725.828434-3-mario.limonciello@amd.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>