i2c: busses: i2c-st: Fix copy/paste function misnaming issues

Fixes the following W=1 kernel build warning(s):

 drivers/i2c/busses/i2c-st.c:531: warning: expecting prototype for st_i2c_handle_write(). Prototype was for st_i2c_handle_read() instead
 drivers/i2c/busses/i2c-st.c:566: warning: expecting prototype for st_i2c_isr(). Prototype was for st_i2c_isr_thread() instead

Fix the "enmpty" typo while here.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>