GCC compiler can generate 8-bit stores when modifying bitfields of
volatile structs (https://github.com/espressif/esp-idf/issues/597).
In the specific case of I2C driver, this resulted in byte_num field
to be written using s8i. However the peripheral requires 32-bit
writes, and ignores 8-bit writes. This change modifies the code to
compose the 32-bit command register value first, and then write the
complete value to the hardware.