drivers/modem: Increase sprintk buffers to hold parameter types

These drivers use stack buffers to hold AT command strings which are
generated at runtime using sprintk. The buffers are only sized for the
expected range of values, not the full possible range given the datatypes
involved. Values outside this expected range could cause a buffer overflow.

To mitigate this, increase the size of each buffef to hold the full range
of each parameter type.

Signed-off-by: Keith Packard <keithp@keithp.com>