arch/amebad/amebad_serial.c: fix wrong size of tx buffer for uart2
arch/amebad/amebad_serial.c: fix wrong size of tx buffer for uart2

Uart2 uses tx buffer for g_uart2txbuffer global variable and
its definition is
static char g_uart2txbuffer[CONFIG_UART2_TXBUFSIZE];.
So, g_uart2port structure should have CONFIG_UART2_TXBUFSIZE in the .xmit->.size,
not CONFIG_UART1_TXBUFSIZE.
This commit fixes above.

Signed-off-by: sunghan-chang <c954e25285721d8faa8b1a5c1ae2ef5a35338d32@samsung.com>

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