[DRIVERS] serial: Few improvements to 8250-uart

This patch does two improvements to 8250-uart drivers:
1. The function uart_8250_clear_errors() is only required
when UART_POLLING is disabled.
2. Do not break poping data from Rx FIFO when Rx buffer
is full. If Rx buffer is full then drop data instead of
breaking loop.

Signed-off-by: Anup Patel <anup@brainfault.org>