drivers: i2c: Refactor Atmel SAM (TWIHS) driver
drivers: i2c: Refactor Atmel SAM (TWIHS) driver

Various minor changes to the Atmel SAM (TWIHS) driver
- clean up variable names, comments
- refactor i2c_clk_set function
- do not save configuration in dev_data
- improve ISR NACK handling
- print "Device initialized" string

Signed-off-by: Piotr Mienkowski <be7d9ab625d3cb5e3a4a3202f3529b954dba084c@gmail.com>

mbolivar/zephyr,kraj/zephyr,nashif/zephyr,punitvara/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,aceofall/zephyr-iotos,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyriot/zephyr,mbolivar/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyriot/zephyr,punitvara/zephyr,punitvara/zephyr,finikorg/zephyr,explora26/zephyr,zephyriot/zephyr,finikorg/zephyr,explora26/zephyr,ldts/zephyr,finikorg/zephyr,kraj/zephyr,ldts/zephyr,Vudentz/zephyr,ldts/zephyr,zephyriot/zephyr,Vudentz/zephyr,kraj/zephyr,Vudentz/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,mbolivar/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,kraj/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,aceofall/zephyr-iotos,zephyriot/zephyr,aceofall/zephyr-iotos