stm32/main: Increase default UART REPL rx buffer from 64 to 260 bytes.

This allows the UART to buffer at least 256 bytes (taking into account the
extra byte needed by the ring buffer, and word alignment).

ports/stm32/main.c

@@ -78,7 +78,7 @@ STATIC fs_user_mount_t fs_user_mount_flash;
 
 #if defined(MICROPY_HW_UART_REPL)
 #ifndef MICROPY_HW_UART_REPL_RXBUF
-#define MICROPY_HW_UART_REPL_RXBUF (64)
+#define MICROPY_HW_UART_REPL_RXBUF (260)
 #endif
 STATIC pyb_uart_obj_t pyb_uart_repl_obj;
 STATIC uint8_t pyb_uart_repl_rxbuf[MICROPY_HW_UART_REPL_RXBUF];