drivers: modem: delay after each AT command

Change logic in `modem_cmd_handler_setup_cmds` and
`modem_cmd_handler_setup_cmds_nolock` to always sleep
after each sent AT command.

The issue was found using a logic analyzer while debugging 'NO CARRIER'
from a BG95 modem while being in data mode.
The reason 'NO CARRIER' occured was due to crosstalk to DTR which
terminated data mode.

The logic analyzer also revealed that without the delay the next AT
command is sent before the whole 'OK\r\n' is received. This is not
addressed in this commit.

Fixes https://github.com/zephyrproject-rtos/zephyr/issues/47082



Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>