drivers: serial: nrf: Fix interrupts enabling/disabling

This patch corrects the way the INTENSET and INTENCLR registers are
accessed. When these registers are written with a bitmask, specified
bits are set or cleared in the INTEN (interrupt enabling) register,
so there is no need to read the previous state of these registers
(when they are read, the current state of INTEN is returned).
For INTENSET this patch eliminates only one unnecessary read, but for
INTENCLR the change is crucial because in the previous version any
write to the register disabled all the enabled interrupts.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>