uart/ns8250: Disable interrupts sooner

Disable interrupts before we set the parameters for the UART. Usually,
it makes no difference, but it's possible that setting the baud rate, etc
could create problems if there's data pending, so move the interrupt
disabling ealier.

Sponsored by: Netflix
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D47075