rtc: cmos: prevent kernel warning on IRQ flags mismatch

The Microsoft Surface 3 tablet shares interrupt line between RTC and one of SPI
controllers. However, the rtc_cmos driver doesn't allow shared interrupts and
user sees the following warning

genirq: Flags mismatch irq 8. 00000080 (8086228E:02) vs. 00000000 (rtc0)
...
[<ffffffffa004eb01>] pxa2xx_spi_probe+0x151/0x600 [spi_pxa2xx_platform]

Allow RTC driver to use shared interrupts.

Seems we are on the safe side to do just this simple change since
cmos_interrupt() handler checks for the actual hardware status anyway.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>