drivers: modem: modem receiver is sending extra bytes around \r\n

[rerickson1]

Describe the bug
Extra 0's are being added before and after the '\r\n' terminator of every at command sent.
I am using an NRF52840 host processor to communicate with an HL7800 modem.

To Reproduce
Steps to reproduce the behavior:
This should be easily reproducible using the WNC modem driver with the NRF52840.

Expected behavior
Extra bytes should not be inserted into the messages being sent on the UART.

Impact
Showstopper for continuing my driver development.

Screenshots or console output

Environment (please complete the following information):

• OS: Windows 10
• Toolchain: GCC 7_2018-q2-update
• Commit SHA or Version used: Zephyr 5f2099f

@mike-scott

[mike-scott]

@rerickson1 Confirming, that with my analyzer, I'm also seeing NULL characters sent to the wire. Oddly, this doesn't break communications on the WNC-M14A2A modem. So, I didn't notice it during my testing.

I'll submit a fix shortly.

NOTE: Love that Saleae UI.