[efi] Fix operator precedence in autoexec network download

[efi] Fix operator precedence in autoexec network download

The != operator has higher precedence than = in C, so the expressions:

  rc = imgacquire ( ..., image ) != 0

are parsed as:

  rc = ( imgacquire ( ..., image ) != 0 )

This assigns the boolean result (0 or 1) to rc instead of the actual
return code from imgacquire().  As a result, strerror(rc) reports an
incorrect error message when debugging is enabled.

Add parentheses around each assignment to ensure rc captures the
actual return value, matching the pattern already used in
efi_autoexec_filesystem() within the same file.

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>