serial: pic32: Add checks for devm_clk_get() in pic32_uart_probe()

As the devm_clk_get() may return ERR_PTR, its return value needs to be
checked to avoid invalid poineter dereference.

Signed-off-by: Yuan Can <yuancan@huawei.com>
Link: https://lore.kernel.org/r/20221125093832.33386-1-yuancan@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>