oemcommands: fix invalid string find result

Comparison on a std::string::find was against -1, but find returns
an unsigned size_t.  The correct approach is to compare against npos.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5a5aeff0dad9d2d47eef9bdb258643df8c4464bf
1 file changed