drivers: modem: sara-r4: make room for NUL in match_buf

In order to still process 128 bytes at a time, let's add 1 to the
length of match_buf which will be used to store a NUL char when
the match_buf is at max size (size - 1).

Otherwise we're processing 127 bytes at a time which is inefficient
compared to the data buffer sizes in net_buf.

Signed-off-by: Michael Scott <mike@foundries.io>