pwm: pwm_sifive: Fix LOG Kconfig typo
pwm: pwm_sifive: Fix LOG Kconfig typo

LOG_LEVEL should be set to CONFIG_PWM_LOG_LEVEL not
CONFIG_LOG_PWM_LEVEL. In cleaning this up use
LOG_MODULE_REGISTER(x,y) form to reduce 2 lines to 1.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,ldts/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,nashif/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr