serial: 8250-mtk: Use platform_get_irq_optional() for optional irq

serial: 8250-mtk: Use platform_get_irq_optional() for optional irq

As platform_get_irq() now prints an error when the interrupt does not
exist, this warnings are printed on bananapi-r2:

[    4.935780] mt6577-uart 11004000.serial: IRQ index 1 not found
[    4.962589] 11002000.serial: ttyS1 at MMIO 0x11002000 (irq = 202, base_baud = 1625000) is a ST16650V2
[    4.972127] mt6577-uart 11002000.serial: IRQ index 1 not found
[    4.998927] 11003000.serial: ttyS2 at MMIO 0x11003000 (irq = 203, base_baud = 1625000) is a ST16650V2
[    5.008474] mt6577-uart 11003000.serial: IRQ index 1 not found

Fix this by calling platform_get_irq_optional() instead.

now it looks like this:

[    4.872751] Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled

Fixes: 7723f4c5ecdb8d83 ("driver core: platform: Add an error message to platform_get_irq*()")
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191027062117.20389-1-frank-w@public-files.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>