Bluetooth: btnxpuart: Fix missing devm_request_irq() return value check

Return value of devm_request_irq() must be checked (function is even
annotated) and without it clang W=1 complains:

  btnxpuart.c:494:6: error: unused variable 'ret' [-Werror,-Wunused-variable]

Setting up wakeup IRQ handler is not really critical, because the
handler is empty, so just log the informational message so user could
submit proper bug report and silences the clang warning.

Fixes: c50b5666 ("Bluetooth: btnxpuart: Implement host-wakeup feature")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>