iscsi_ibft: Fix missing break in switch statement

Add missing break statement in order to prevent the code from falling
through to case ISCSI_BOOT_TGT_NAME, which is unnecessary.

This bug was found thanks to the ongoing efforts to enable
-Wimplicit-fallthrough.

Fixes: b33a84a3

 ("ibft: convert iscsi_ibft module to iscsi boot lib")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>