msm_serial_hs: Increase rx buffer size to 1024bytes

Increase the UARTDM rx buffer size from 512 bytes
to 1024 bytes. This patch helps to increase the
rx data rates of the clients that uses serial_hs driver.

CRs-Fixed: 514385
Change-Id: I2956020af299dda849cfbef795728da2d8664c77
Signed-off-by: Dilip Kota <c_dkota@codeaurora.org>

Conflicts:
	drivers/tty/serial/msm_serial_hs.c