brcmfmac: fix invalid access to struct acpi_device fields

The fields of struct acpi_device are only known when CONFIG_ACPI is
defined. Fix this by using a helper function. This will resolve the
issue found in linux-next:

 ../brcmfmac/bcmsdh.c: In function 'brcmf_ops_sdio_probe':
 ../brcmfmac/bcmsdh.c:1139:7: error: dereferencing pointer to incomplete type
   adev->flags.power_manageable = 0;
       ^

Fixes: f0992ace

 ("brcmfmac: prohibit ACPI power management ...")
Cc: Fu, Zhonghui <zhonghui.fu@linux.intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>