rtc: mpfs: fix unchecked devm_clk_get() error pointer in probe()

devm_clk_get(&pdev->dev, "rtcref")'s return value was passed straight
into clk_get_rate() without checking it for an error first, unlike the
"rtc" clock a few lines above which is correctly checked with
IS_ERR(). clk_get_rate() only guards against a NULL clk, not an error
pointer:

	if (!clk)
		return 0;
	...
	rate = clk_core_get_rate_recalc(clk->core);

so if devm_clk_get() ever returns an error pointer here (for example
ERR_PTR(-EPROBE_DEFER), which is the normal, expected outcome if the
clkcfg clock-provider this RTC depends on has not registered its
clocks yet by the time this driver probes), clk_get_rate() dereferences
that error pointer instead of returning 0, crashing instead of letting
probe defer.

Capture the clock in the existing 'clk' local and check it with
IS_ERR() before calling clk_get_rate(), matching the handling already
used for the "rtc" clock in this same function.

Signed-off-by: Manush Prajwal <manushprajwal555@gmail.com>
Fixes: 0b31d703598d ("rtc: Add driver for Microchip PolarFire SoC")
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/6a9d4b02.7d74b517.206c4c.75f8@mx.google.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>