drivers: modem: skip quoted delimiters

Given the following response: `+CIPSTA:ip6ll:"FE80::EDC:7EFF:FEDD:110C"`

The response delimiter is `:`, but there is also a quoted string that
contains the delimiter character. These delimiters should not be
considered when searching for the end of a parameter.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>