shell: Make TX mutex timeout configurable

The 50 ms TX mutex timeouts introduced in commit b0a0febe cause
dropped messages when used with the shell log backend. This
commit makes time timeout configurable using the
CONFIG_SHELL_TX_TIMEOUT_MS Kconfig option and makes the
default timeout longer when shell log backend is enabled.

Link: #90215

Signed-off-by: Tommi Kangas <tommi.kangas@nordicsemi.no>