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 <piotr.mienkowski@gmail.com>