ARM: pxa: Make id const in pwm_probe()

This fixes the compiler warning:

	arch/arm/plat-pxa/pwm.c: In function 'pwm_probe':
	arch/arm/plat-pxa/pwm.c:179: warning: initialization discards qualifiers from pointer target type

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>