serial: 8250_mtk: Fix UART_EFR register address

commit bb0b197a upstream.

On MediaTek SoCs, the UART IP is 16550A compatible, but there are some
specific quirks: we are declaring a register shift of 2, but this is
only valid for the majority of the registers, as there are some that
are out of the standard layout.

Specifically, this driver is using definitions from serial_reg.h, where
we have a UART_EFR register defined as 2: this results in a 0x8 offset,
but there we have the FCR register instead.

The right offset for the EFR register on MediaTek UART is at 0x98,
so, following the decimal definition convention in serial_reg.h and
accounting for the register left shift of two, add and use the correct
register address for this IP, defined as decimal 38, so that the final
calculation results in (0x26 << 2) = 0x98.

Fixes: bdbd0a7f

 ("serial: 8250-mtk: modify baudrate setting")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220427132328.228297-2-angelogioacchino.delregno@collabora.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>