fsutils/flashtool: Fix compile error due to missing statement after default label

[tuansuper]

In flashtool_main.c, the 'default:' label in a switch statement was
followed directly by '}', which is invalid in C and causes a compile
error.

This commit adds a 'break;' statement after the default label to
resolve the error.

Impact:

• Build: Fixes compilation of flashtool utility
• Runtime: No functional change
• Area: fsutils/flashtool

Tested:

• Build Host: Ubuntu 22.04, arm-none-eabi-gcc 15.10.3
• Target: sim:nsh (compilation test)

[Abhishekmishra2808]

Thanks - @tuansuper, please sign your commit message with git commit -s
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