sdcard: use FUSE_BIG_WRITES for FUSE writes

Slightly optimizes the writes used by sdcard to increase
throughput and decrease cpu load. Update the read
size to 256 x 1024 + 128 from current 8192 bytes since
writes can go as high as that.

Change-Id: I3bad425f31d4aa6f44f546e3d31439fd5bdca9ea
Signed-off-by: Sundar Raman <sunds@ti.com>