shell: backend: telnet: Rename write and read functions

Rename write() to telnet_write(), and read() to telnet_read() so
that if we enable CONFIG_POSIX_API, the compiler will not complain
about those two functions as they conflict with POSIX API ones.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>