gdb/testsuite/gdb.rocm: Fix incorrect use of continue N in multi-inferior-gpu.exp

gdb/testsuite/gdb.rocm: Fix incorrect use of continue N in multi-inferior-gpu.exp

The gdb.rocm/multi-inferior-gpu.exp testcase uses a "continue $thread"
command, but this is incorrect.  If "continue" is given an argument, it
sets the ignore count of the breakpoint the thread stopped at.

For this testcase it does not really matter since the breakpoint is not
meant to be hit anymore, so whatever the ignore count is won't influence
the outcome of the test.  It is worth fixing nevertheless.

Change-Id: I0eb674d5529cdeb9e808b74870a29b6077265737
Approved-By: Simon Marchi <simon.marchi@efficios.com>