sfxge(4): fix PreFAST static analysis warning (C6001)

Fix warning
"C6001: Using uninitialized memory '*sensor_maskp'"
which could occur when the npages argument to efx_mcdi_sensor_info()
is less than or equal to zero.

Submitted by: Andrew Lee <alee at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18128