ALSA: atmel_abdac: clk_round_rate() can return a zero upon error

Treat both negative and zero return values from clk_round_rate()
as errors.  This is needed since subsequent patches will convert
clk_round_rate()'s return value to be an unsigned type, rather
than a signed type, since some clock sources can generate rates higher
than (2^31)-1 Hz.

Eventually, when calling clk_round_rate(), only a return value of
zero will be considered a error; all other values will be
considered valid rates.  The comparison against values less than
0 is kept to preserve the correct behavior in the meantime.

Signed-off-by: Paul Walmsley <pwalmsley@nvidia.com>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>