ACPI: bgrt: Use sysfs_emit

sysfs_emit is preferred to snprintf for emitting values after
commit 2efc459d ("sysfs: Add sysfs_emit and sysfs_emit_at to format
sysfs output").

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>