drivers: modem: fix race condition when waiting on cmd reply

Command was sent first, then a semaphore was reset. This semaphore could
be released by received reply even before semaphore was reset. This is
quite unlikely, but possible result of race condition.

Move k_sem_reset() call before attempt to send command over modem
interface. This makes sure that receiving reply momentarily after
sending request will always be handled properly.

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