spi: spi_qsd: Don't connect/disconnect BAM pipe after every transfer

The driver currently makes a call to disconnect then reconnect the BAM
pipes after every transfer in a bid to flush the BAM pipes.
Do this only if the BAM transfer somehow failed, else it will incur
additional overhead per transfer.

Change-Id: I9bc3a367042320e5364cb34eae2df45fcb79cc70
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>