cmd_i2c: bugfix: add missing brace

The sub-command parser missed a brace, so "return 0;" is always
taken and no error message is diplayed if you say "i2c scan"
instead of "i2c probe", for example.

Proper brace is added. Also, a misleading and unneeded else
is removed.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com.it>