i2c: i2c-bfin-twi: Always access 16 bit MMR by bfin 16 bit access Macro.

Otherwise, the compiler may optimize assemble code into 8bit access instruction
when target register is 8 bit width.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>