pwm: pxa: Remove clk_enabled field from struct pxa_pwm_chip

clk_enable/clk_disable maintain an enable_count, clk_prepare and clk_unprepare
also maintain a prepare_count. These APIs will do prepare/enable when the first
user calling these APIs, and do disable/unprepare when the corresponding counter
reach 0. Thus We don't need to maintain a clk_enabled counter here.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>