firmware: iscsi_ibft: fix ISCSI_IBFT Kconfig entry

Fix ISCSI_IBFT Kconfig entry, replace tab with a space character.

Fixes: 138fe4e0 ("Firmware: add iSCSI iBFT Support")
Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>