drivers: modem: cmd_handler: honor num of params in parse_params

Modem commands are setup with arg_count to denote the number of params
to parse before returning.  Let's honor that setting and return once
we've parse them.

This fixes a run-on bug where the parser would find as many of the
parameters via the supplied delimiter as it could.

Signed-off-by: Michael Scott <mike@foundries.io>