fmu-v5 defconfig: increase USART3 from 1500 to 3000

This is needed for companions with high baudrate and high data rate.
Tested with 1500000 Baudrate and mavlink TX rate of ~120KB/s: no drops.

I did not test the exact limit, something like 2500 might be enough. But
we (still) have enough free RAM on FMU-v5.

platforms/nuttx/nuttx-configs/px4fmu-v5/nsh/defconfig

@@ -871,7 +871,7 @@ CONFIG_USART2_OFLOWCONTROL=y
 # USART3 Configuration
 #
 CONFIG_USART3_RXBUFSIZE=600
-CONFIG_USART3_TXBUFSIZE=1500
+CONFIG_USART3_TXBUFSIZE=3000
 CONFIG_USART3_BAUD=57600
 CONFIG_USART3_BITS=8
 CONFIG_USART3_PARITY=0