rt2x00: fix clk_get call

clk_get() takes two arguments and might return ERR_PTR(), so we
have to nullify pointer on that case, to do not break further call
to clk_get_rate().

Reported-by: Felix Fietkau <nbd@nbd.name>
Fixes: 34db70b9 ("rt2x00: add copy of clk for soc devices")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>