clk: bcm: rpi: Fix error handling of raspberrypi_fw_get_rate

The function raspberrypi_fw_get_rate (e.g. used for the recalc_rate
hook) can fail to get the clock rate from the firmware. In this case
we cannot return a signed error value, which would be casted to
unsigned long. Fix this by returning 0 instead.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/20220625083643.4012-1-stefan.wahren@i2se.com
Fixes: 4e85e535

 ("clk: bcm283x: add driver interfacing with Raspberry Pi's firmware")
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>