cmd: pwm: Rework argc sanity checking

cmd: pwm: Rework argc sanity checking

Currently, we check argc in a number of places to make sure that we have
all of the required arguments for each of the pwm sub-commands.
However, there's at least one place where we've got dead code as we'll
never have argc == 0, due to checking that argc was at least 4 earlier
and having only subtracted 3.  Rework things so that when we have
determined our subcommand make sure we have the right number of
arguments for it, or error out.  This means we can stop checking against
argc again later.

Reported-by: Coverity (CID: 316601)
Cc: Pragnesh Patel <pragnesh.patel@sifive.com>
Signed-off-by: Tom Rini <trini@konsulko.com>