tty: xuartps: Don't write IRQ disable register to enable interrupts
A comment states, that, according to the data sheet, to enable
interrupts the disable register should be written, but the enable
register could be left untouched. And it suspsects a HW bug requiring
to write both.
Reviewing the data sheet, these statements seem wrong. Just as one would
expect. Writing to the enable/disable register enables/disables
interrupts.
Hence the misleading comment and needless write to the disable register
are removed from the enable sequence.