ddr: altera: n5x: size_t cannot be less than 0

ddr: altera: n5x: size_t cannot be less than 0

The function socfpga_get_handoff_size returns an int so make the struct
fields used to accept the return value also an int so that testing for
less than 0 is then valid.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>