MIPS: TXx9: use IS_BUILTIN() for CONFIG_LEDS_CLASS

When commit b27311e1 ("MIPS: TXx9: Add RBTX4939 board support")
added board support for the RBTX4939, it added a call to
led_classdev_register even if the LED class is built as a module.
Built-in arch code cannot call module code directly like this. Commit
b33b4407 ("MIPS: TXX9: use IS_ENABLED() macro") subsequently
changed the inclusion of this code to a single check that
CONFIG_LEDS_CLASS is either builtin or a module, but the same issue
remains.

This leads to MIPS allmodconfig builds failing when CONFIG_MACH_TX49XX=y
is set:

arch/mips/txx9/rbtx4939/setup.o: In function `rbtx4939_led_probe':
setup.c:(.init.text+0xc0): undefined reference to `of_led_classdev_register'
make: *** [Makefile:999: vmlinux] Error 1

Fix this by using the IS_BUILTIN() macro instead.

Fixes: b27311e1 ("MIPS: TXx9: Add RBTX4939 board support")
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18544/


Signed-off-by: James Hogan <jhogan@kernel.org>