firmware/ihex2fw.c: restore missing default in switch statement

firmware/ihex2fw.c: restore missing default in switch statement

Commit 2473238eac95 ("ihex: add support for CS:IP/EIP records") removes
the "default:" statement in the switch block, making the "return
usage();" line dead code and ihex2fw silently ignoring unknown options.
Restore this statement.

This bug was found by building with HOSTCC=clang and adding
-Wunreachable-code-return to HOSTCFLAGS.

Fixes: 2473238eac95 ("ihex: add support for CS:IP/EIP records")
Signed-off-by: Nicolas Iooss <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: David Woodhouse <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>