ichspi: clear byte count in ich_start_hwseq_xfer()

This fixes a regression introduced by a ichspi driver refactor in
`commit 7ed1337309d3fe74f5af09520970f0f1d417399a`

When preparing for a hwseq transfer in ich_start_hwseq_xfer(), clear the
HSFC_FDBC bits before setting the new byte count. Otherwise the old byte
count bits remain set, resulting in too many bytes being transferred.

In particular this causes status register read/write operations to fail,
and probably causes issues in other hwseq operations as well.

BUG=b:257845782,b:258280679
BRANCH=none
TEST=flashrom {--wp-enable,--wp-disable}

Change-Id: I2535bdfb77ff370bddcb507a229bbf4119681cdf
Signed-off-by: Nikolai Artemiev <nartemiev@google.com>
---
M ichspi.c
1 file changed, 25 insertions(+), 0 deletions(-)