xilinx: zynqmp: Do not check 0 as invalid return from snprintf

xilinx: zynqmp: Do not check 0 as invalid return from snprintf

U-Boot SPL on ZynqMP is using CONFIG_SPL_USE_TINY_PRINTF which doesn't
return any return value and all the time returns 0. That's why
even correct snprintf was returning in SPL chip ID as "unknown".
Change checking condition and allow snprintf to return 0 which is according
manual patch successful return.
"If an output error is encountered, a negative value is returned."

Fixes: 43a138956f7e ("arm64: zynqmp: Get rid of simple_itoa and replace it by snprintf")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>