pwm: tiehrpwm: inline accessor functions

These elementary functions should be inlined for fastest access. Also
fixes this warning as a side-effect (when no PM_SLEEP is selected):

drivers/pwm/pwm-tiehrpwm.c:141:12: warning: 'ehrpwm_read' defined but not used [-Wunused-function]

Signed-off-by: Wolfram Sang <wsa@sang-engineering.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>