drivers: sensors: explorir_m: fix uart flush early termination

`explorir_m_uart_flush_until_end` terminates immediately if the read
variable is initially equal to `EXPLORIR_M_MAX_RESPONSE_DELAY` and
`uart_poll_in` does not read anything.

Fix this by only checking the read variable if `uart_poll_in` reads a char.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>