shell: mqtt: Avoid using POSIX function names

This commit fixes this error seen in CI so that things
work even if CONFIG_POSIX_API is enabled.

subsys/shell/backends/shell_mqtt.c:727:12: error:
  conflicting types for 'write'; have
 'int(const struct shell_transport *, const void *, size_t,  size_t *)'
  727 | static int write(const struct shell_transport *transport,
                         const void *data, size_t length)

include/zephyr/posix/unistd.h:230:9: note: previous declaration
 of 'write' with type
 'ssize_t(int,  const void *, size_t)'
  230 | ssize_t write(int file, const void *buffer, size_t count);

subsys/shell/backends/shell_mqtt.c:787:12: error:
 conflicting types for 'read'; have
 'int(const struct shell_transport *, void *, size_t,  size_t *)'
  787 | static int read(const struct shell_transport *transport,
                        void *data, size_t length, size_t *cnt)

include/zephyr/posix/unistd.h:231:9: note: previous declaration
 of 'read' with type
 'ssize_t(int,  void *, size_t)'
  231 | ssize_t read(int file, void *buffer, size_t count);

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