i2c: rx <-> tx typo's, NULLing free'd variable, consistent CRITICAL sects

i2c: rx <-> tx typo's, NULLing free'd variable, consistent CRITICAL sects

A couple of typos referencing tx_ring_buf when rx_ring_buf, slv_tx_mux
instead of slv_rx_mux.

Also, I2C_ENTER_CRITICAL()/I2C_EXIT_CRITICAL() usage was not consistent.
Only some of the _set_ functions had them.  Most of the _get_ function
had them?  It is my understanding that they should be wrapped around
writes, not reads? (I think we still need the lock for reading pairs of consistent values)

Also, the ticks_to_wait timeout handling in i2c_master_cmd_begin() would
not handle integer rollover correctly.

Merges https://github.com/espressif/esp-idf/pull/1180