scsi: target: iscsi: Remove unused variable xfer_len

The variable 'xfer_len' is being initialized and incremented but it is
never actually referenced in any other way. The variable is redundant and
can be removed.

Cleans up clang scan build warning:
drivers/target/iscsi/iscsi_target_erl1.c:586:45: warning: variable
'xfer_len' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20240307104553.1980860e

-1-colin.i.king@gmail.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>