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.

c590b3545ac5ad37010d25e84e24ac94a6539205 was integrated in
https://github.com/zephyrproject-rtos/zephyr/pull/39041

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