uart_esp32: alter the uart.h include order to avoid warning

The esp-idf package contains on the BSP a definition of the BIT macro
unconditionally #defined when rom/uart.h is #included. When Zephyr's
uart.h (which pulls in include/misc/util.h) that conditionally

This generates a compile warning on the re-definiton of BIT by the
esp-idf package.

Thus include uart.h after esp-idf's rom/uart.h to have BIT not
re-defined thanks to Zephyr's conditional definition.

Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>