mmc: omap: Increase number of blks we can handle

The above block layer is doing a "min" of max_req_size
and max_blk_count*512 before submitting each request.

As a result, the maximum size of request we are getting
is only 120*512 = 60KB even though each line in the
ADMA table can handle can handle that data alone.

So, we increase the max_blk_count we can handle. Also,
note that worst case is when above block layer gives
us 512 segments, in which there are 511 single block
entries, but one large block that is of size
mmc->max_req_size - (511*512) bytes.

In this case, we use the reserved 512 table entries
to break up the large request. Also adjust the
max_segs accordingly.

Change-Id: I733effdebea94f51a49bbf323950529afbb00dc4
Signed-off-by: Viswanath Puttagunta <vishp@ti.com>