firmware: meson-sm: Allow 0 as valid return value

Some special SMC calls (i.e. the function used to retrieve the serial
number of the Amlogic SoCs) returns 0 in the register 0 also when the
data was successfully read instead of using the register to hold the
number of bytes returned in the bounce buffer as expected.

With the current implementation of the driver this is seen as an error
and meson_sm_call_read() returns an error even though the data was
correctly read.

To deal with this when we have no information about the amount of read
data (that is 0 is returned by the SMC call) we return to the caller
the requested amount of data and 0 as return value.

Signed-off-by: Carlo Caione <carlo@endlessm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>