risc-v fe310 console driver uart read error

[Alan Cudmore]

In the riscv/riscv fe310_uart_read function, the uart RX data/status register is read twice when reading a character: once for the status, then a second time when reading the data. Reading the RX data/status register twice discards the input data from the first read.

It can be fixed by first reading the RX data/status register into a local variable.

This was verified on a Kendryte K210 CPU which uses the same SiFive? UART. I will submit a patch to fix it.

[Alan Cudmore <alan.cudmore@…>]

In 88f4d44f/rtems:

bsps/riscv/riscv: Fix fe310_uart_read

Note: Resending after learning how to use git send-email, please disregard previous message.

This fixes the riscv fe310 console driver fe310_uart_read function. The function
reads the RX status/data register to check if data is available, but discards
the data and reads it a seconds time.
Also cleared the interrupt enable bit in the first_open function.

Close #4719