drivers: serial: fix uart_sam0 async rx timeout calculation

Zephyr moved to microseconds to represent uart async timeout.
This one has some additional logic that was not adjusted in
the transition from milliseconds.

c590b354 was integrated in
https://github.com/zephyrproject-rtos/zephyr/pull/39041



Signed-off-by: Stefan Giroux <stefan.g@feniex.com>