s390/ipl: Clear SBP flag when bootprog is set

s390/ipl: Clear SBP flag when bootprog is set

With z16 a new flag 'search boot program' was introduced for
list-directed IPL (SCSI, NVMe, ECKD DASD). If this flag is set,
e.g. via selecting the "Automatic" value for the "Boot program
selector" control on an HMC load panel, it is copied to the reipl
structure from the initial ipl structure. When a user now sets a
boot prog via sysfs, the flag is not cleared and the bootloader
will again automatically select the boot program, ignoring user
configuration.

To avoid that, clear the SBP flag when a bootprog sysfs file is
written.

Cc: stable@vger.kernel.org
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>