serial: 8250_bcm2835aux: Restore clock error handling

The commit fcc446c8 ("serial: 8250_bcm2835aux: Add ACPI support")
dropped the error handling for clock acquiring. But even an optional
clock needs this.

Fixes: fcc446c8

 ("serial: 8250_bcm2835aux: Add ACPI support")
Cc: stable <stable@kernel.org>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20231220114334.4712-1-wahrenst@gmx.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>