modpost: fix acpi MODULE_DEVICE_TABLE built with mismatched endianness

When CONFIG_SATA_AHCI_PLATFORM=m, modpost outputs incorect acpi
MODULE_ALIAS() if the endianness of the target and the build machine
do not match.

When the endianness of the target kernel and the build machine match,
the output is correct:

  $ grep 'MODULE_ALIAS("acpi' drivers/ata/ahci_platform.mod.c
  MODULE_ALIAS("acpi*:APMC0D33:*");
  MODULE_ALIAS("acpi*:010601:*");

However, when building a little-endian kernel on a big-endian machine
(or vice versa), the output is incorrect:

  $ grep 'MODULE_ALIAS("acpi' drivers/ata/ahci_platform.mod.c
  MODULE_ALIAS("acpi*:APMC0D33:*");
  MODULE_ALIAS("acpi*:0601??:*");

The 'cls' and 'cls_msk' fields are 32-bit.

DEF_FIELD() must be used instead of DEF_FIELD_ADDR() to correctly handle
endianness of these 32-bit fields.

The check 'if (cls)' was unnecessary; it never became NULL, as it was
the pointer to 'symval' plus the offset to the 'cls' field.

Fixes: 26095a01 ("ACPI / scan: Add support for ACPI _CLS device matching")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>