drivers: modem: verify send semaphore before taking any action

Semaphore argument is checked only after requested command is already
sent over modem interface. It makes little sense to return -EINVAL when
half of the requested operation (send) has already been done.

Check timeout and semaphore arguments just on the beginning of
function. Return -EINVAL early, before sending any data or taking any
locks.

Clear out 'sem' variable when there is no need to wait. Use this
variable later on as an indication to actually wait on semaphore or not.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>