mgmt/mcumgr: Fix UART processing of TX frames
mgmt/mcumgr: Fix UART processing of TX frames

The commit fixes problem with UART TX frame processing, which was
caused by incorrect output buffer size evaulation, where input buffer
of certain len would cause entire output frame to be filled with no
place left for CRC causing a transmission to hang.
The commit, additionally, moves input buffer reminder processing out of
the input buffer processing loop.

Fixes #37893

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr