fm10k: reduce the scope of the local msg variable

The msg variable in the fm10k_mbx_validate_msg_size and
fm10k_sm_mbx_transmit functions is only used within the do {} loop
scope. Reduce its scope only to where it is used.

This was detected by cppcheck, and resolves the following warnings
produced by that tool:

[fm10k_mbx.c:299]: (style) The scope of the variable 'msg' can be reduced.
[fm10k_mbx.c:2004]: (style) The scope of the variable 'msg' can be reduced.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>