arm64: zynqmp: Fix command error values properly

Process errors from command via cmd_process_error() as is done on Versal.
When internal function returns different number then CMD_RET_SUCCESS(0),
CMD_RET_FAILURE(1) or CMD_RET_USAGE(-1) shell react on these errors by
throwing an error like
"exit not allowed from main input shell." that's why use
cmd_process_error() to make sure that error code is all the time correct.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/d511935ba10daf95c70996fae6e6ffc374efffa0.1685618464.git.michal.simek@amd.com