i2c: mt65xx: Avoid double initialization of restart_flag in isr

In the mtk_i2c_irq() handler, variable restart_flag is initialized
to zero and then reassigned with I2C_RS_TRANSFER if and only if
auto_restart is enabled.

Avoid a double initialization of this variable by transferring the
auto_restart check to the restart_flag declaration.

This commit brings no functional changes.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>