Blackfin Serial Driver: disable dma rx interrupt only rather than all irqs

The UART RX handling code isn't exactly speeding, so don't go disabling
all interrupts when processing the buffer.  Just disable the relevant DMA
interrupt.  This greatly improves latency of the system when utilizing the
UART.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>