Store the port so the in use port will be referenced when reopening.

[SimonWilks]

This allows the SF0x lasers to operate on non-default ports and resolves #2004. Tested on both the default port (Serial4) and Telem2:

nsh> sf0x start /dev/ttyS2
nsh> sf0x test
sf0x: single read
sf0x: val:  0.08 m
sf0x: time: 28345363
sf0x: read #0
sf0x: val:  0.080 m
sf0x: time: 28429739
sf0x: read #1
sf0x: val:  0.080 m
sf0x: time: 29103631
sf0x: read #2
sf0x: val:  0.080 m
sf0x: time: 29606865
sf0x: read #3
sf0x: val:  0.080 m
sf0x: time: 30109990
sf0x: read #4
sf0x: val:  0.080 m
sf0x: time: 30613180
sf0x: PASS
nsh> sf0x test
sf0x: single read
sf0x: val:  0.40 m
sf0x: time: 50787312
sf0x: read #0
sf0x: val:  0.400 m
sf0x: time: 50870123
sf0x: read #1
sf0x: val:  0.400 m
sf0x: time: 50953459
sf0x: read #2
sf0x: val:  0.400 m
sf0x: time: 51457711
sf0x: read #3
sf0x: val:  0.370 m
sf0x: time: 51961960
sf0x: read #4
sf0x: val:  0.120 m
sf0x: time: 52466071
sf0x: PASS
nsh> 

[LorenzMeier]

Thanks!