mmc: dw_mmc: Increase timeout to 4 minutes (as in Linux kernel)

The commit: d9dbb97b
"mmc: dw_mmc: Zap endless timeout" removed endless loop waiting for end
of dw mmc transfer.

For some workloads - dfu test @ Odroid XU3 (sending 8MiB file) -
and SD cards (e.g. MicroSD Kingston 4GiB, Adata 4GiB)
the default timeout is to short.

The new value - 4 minutes (240 seconds) - is the same as the one used in
Linux kernel driver. Such fix should be good enough until we come up
with better fix for this issue.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@konsulko.com>
Tested-by: Przemyslaw Marczak <p.marczak@samsung.com>